How to Make Colors: The red/green/blue columns are a little confusing, so for some people it might be easier to just stab around until you get the color you want. However, for verbal explanation-loving people:
--You have to pick one box in each of the red, green, and blue columns in order to combine them into the color you actually use in the art. I'm going to call this the result color to distinguish it from the red, green, and blue column boxes.
--The result color is shown in the bigger box on top of the columns.
--When you open a document, the result color will always start out as black (lowest box in each column). The art kit will not remember any color combinations you used before you closed the document--once you close and reopen, it will always reset to black.
--The result color is not the color you would get if you mixed the colors of paint you see in the columns. The idea is that you are mixing red, blue, and green light, like a television does to get all the colors you see on it. Colored light works differently than colored paint.
--The higher up the column you click, the more of that color light will be in the result color. This means the result color will be brighter/paler if you choose boxes near the top, and darker/less saturated if you choose boxes near the bottom.
--The higher one column is in relation to the other columns, the more of that color will be in the result color. So if you pick a higher blue and lower red and green, the result color will be somewhere in the blue family.
--If you pick boxes that are next to each other (however high or low they are in the stack), you will end up with some kind of gray. If you pick all the top boxes the result color will be white; if you pick all the bottom boxes it will be black.
--You can get bright primary yellow by choosing the top red and green boxes and the bottom blue box. If you want to make a lighter, softer yellow, go a little higher in the blue column. For a darker, mustardy yellow, bring the red or green down a bit.
--You get bright, primary red by using the top of the red column and the bottom of the other two columns. Same for blue or green (except that green is technically a secondary color).
--You get purple by going high in the red and blue columns and low in the green.
--To get orange, go to the top of the red column and about halfway down the green, leaving the blue at rock bottom.
--For brown, put the red column somewhere in the middle, the green column a few up from the bottom, and the blue only a little up from the bottom.
--For a nice cream color (I like to use this instead of white a lot of the time), click the top of the red and green and one down from the top of the blue. To darken it, move everything down one, making sure the red and green are equal and the blue is one box lower.
Added (2013-03-03, 9:01 AM)
Tips for Using Colors:
--If you're making a rump art to sell at the painter, you want to test the colors on several different colors of horse. You can make a rump art look raised or indented by making it a similar color to the horse but one or two shades lighter or darker, especially if you add a shadow or highlight.
--If you're making a solid-colored object with shadows and highlights, it often works to pick the basic result color and then move all the bars 2 or 3 up for the highlight and 2 or 3 down for the shadows--2 if you want a fairly subtle result, 3 for bolder. I've noticed that a lot of MockUp's winning contest entries use this three-shade technique.
--If you want colors that are subtler or more natural-looking, make sure none of the columns are at rock bottom (this is not a hard and fast rule, especially if you're using dark dull brownish-reds--you can get those by leaving the green and blue at bottom and going in the lower half of the red column). Also, avoid using pure black or pure white unless you want to showcase how extremely black or white that bit of the picture is.
Added (2013-03-03, 9:58 AM)
Art contest tips:
--Remember that the picture voters see is going to be about the size it is on your profile or ranch sign. The size at which you view a picture can make a huge difference in what it looks like, since certain kinds of lines and color combinations can be magnified or made nearly invisible when you view the picture at rump art size. The only sure way to know how an entry will look to voters is to spend 10k to get it approved and then put it in your profile or ranch sign, but you can make a guess by looking at the editable version in the kit and then dragging the game window smaller until the art is a couple of inches tall and wide.
--It helps to guess what other people will be doing, and then do something a little different so your entry will stand out. If the theme is fruit, you might guess that other people will make apples (because they're popular and also associated with horses) and therefore decide on a papaya or a kiwi slice instead. You could even choose a green apple instead of the more expected red. Or you could just make the best red apple out there, but this is risky unless you are very confident that you can make a better apple than anyone else. This is not necessarily linked to general artistic skill, by the way--you may have an excellent understanding of apples even if you're not generally confident with the art kit, or you may be good with the art kit but have a hard time with machine parts or furry textures or something. If the theme is a particular kind of animal, you might want to show the animal in an unusual pose or from an unusual angle.
--Unless you have a really clear picture in your mind, you may want to use a photo or drawing as a source. The source should ideally be squareish (unless you don't plan to use the whole space), clearly visible, and not too complex. Or, rather, it can be complex, but it should be something that you can figure out how to simplify, something that won't suffer from losing a lot of detail.
--When choosing your source picture, you probably want to use an image search on a search engine. If you don't want to be using the same picture as everyone else, don't pick the first few results that come up. You can also search for a particular type that not everyone would know about--if the theme is rabbits, you might look up a list of rabbit breeds and then do an image search for Rexes or Flemish Giants.
--A lot of people will be annoyed if you use the same picture or template in more than one contest. This may be overbalanced if it's a really fantastic picture, or if a lot of different people are voting than voted the other time(s) you used it.
--Consider the level of detail that you want in your picture. If you want or need to use a lot of detail (for example, an animal with a complicated body and many stripes or spots) you might want to only draw one part of it (like a face)--or you could take advantage of the way that drawings tend to look more detailed when they're smaller, and draw the whole thing small in just one part of the available space.
--Tempting as it may be to draw more than one of something in the same picture (like two stallions fighting or two cats cuddling) it'll usually take up too many bytes unless you can make them very simple.
--An easy way to get more precise detail into a picture without going over the byte limit is to make a black (or any other color but white) silhouette.
--If you want to convey an overall pattern (like spots or scales) on an animal or plant, but don't have the bytes left to cover the whole thing with pattern, you can often get away with just putting the pattern in one or two bits of the animal, usually near the upper middle of a largish space like the neck, trunk, or thigh--the same places where you might put a small highlight.
Added (2013-03-04, 1:49 PM)
How the art kit works; or, why there's no eraser tool
There are two basic types of computer program that let you draw pictures--pixel-based and vector-based. Horse Isle 2's art kit is vector-based.
Pixel-based is probably the easiest to understand because it works on the same principle as drawing or painting: you have a tool that makes marks, and you use it on a blank rectangle. You can erase easily--there's a tool that just takes away any pixels under it, leaving the space blank.
Vector-based has no exact physical equivalent--the closest example I can think of is building your own custom bubble-blowing wands, those little loops-on-a-stick that you dip into soapy water. You don't build the bubble itself, you just build the stick and include instructions on what color soapy water to use. In vector art, the program doesn't remember the color of each tiny dot on the page--it just remembers the outlines of shapes and what color to fill them with.
And that is why you can't erase in vector art. Can you erase just part of a soap bubble? No, the whole thing will pop. In more complex vector-based programs like Illustrator, objects are layered on top of each other and you can select one (that's like grabbing hold of the bubble wand) and do things like move it, throw it away, copy it, resize it, combine it with another object, or change where it is in the stack. But the art kit is just a simple little minigame within a game, and we don't get even that advantage.
So, if pixel-based programs make the most intuitive sense, and actually have erasers, and we don't even get the advantages that are in most vector art programs, why on earth did they make the art kit vector-based? One word: memory.
Say I want to draw a blue circle 5 inches tall. The vector-based program has to remember the outline of the circle, where it is on the page, and the color it's filled with. The pixel-based program, on the other hand, has to remember the color (or lack of color) for every one of the thousands of tiny bits of space on the page. So a document containing the same 5-inch blue circle can take up tens or even hundreds of times the space on the hard drive or server.
Here's another analogy: if it were a person mailing a paper copy of the blue circle to you, the vector-based program would send it in a stiff envelope and charge you a dollar or two for postage. The pixel-based program would glue it into an entire glass display case within a crate within another crate, and you'd have to pay a hundred dollars or more. Moreover, once you got it, the display case would take up a lot of room in your home or work space, and you'd trip over it, and it might even get broken.
There are 2 main reasons to reduce the amount of memory an online game takes up--money and speed. I'm a little hazy on the specifics of website building and management, but in general, the more space a game takes up, the more expensive it is for the owners, and they will pass that cost on to the players. The other issue is that a program that takes up a lot of memory and processing space will make the game run much, much slower. You know how you can be coasting along on a small deserted island at 20 fps, and you jump to big crowded Plains and suddenly you're crawling along at 5 fps? A vector-based art kit not only gives you the opportunity to store more pictures, but it won't make the larger game crash to a halt.
At this point you might say, "Well, HI1 had a pixel-based art kit and they made that work." HI1 only stored up to three pictures in each player's account memory. With the HI2 art kit, the game not only has to store up to ten current pictures for each player with an art kit, but it also stores every rump art that was ever placed on a horse that is still in the game. Far more players make rump arts than made art on HI1, and that's not even including the contest entries, five more of which are stored forever every three days. Also, everyone on HI2 is on the same two servers. In HI1, they just make a new server whenever the number of players gets past a certain (fairly low) point, and each server only has to remember the last three pictures that its own players made.
[I'm probably making some kind of terrible error by confusing storage memory with working memory or something like that--if so I hope someone will correct me.]