Part of the problem is that you have some colors that are very similar to each other. For example, on the front of the shirt there's one color that is R36, G106, B54 and another color that is R35, G105, B54. These two colors are too similar for people to see the difference between them. Just pick one of them to use as a replacement for the other. There's actually many different colors on the front of the shirt, like R34, G107, B53 and R33, G106, B53.

There's also colors like R53, G53, G53 and R54, G54, G54 used in the shadow of the pants and the highlight of the shoes.

When you drew this, did you have the anti aliasing option turned on? For pixel art things like that shouldn't be used. I'm not sure why else you would have so many colors that are so similar. I don't really have experience indexing colors because I always use photoshop's "save for web" option and I save things as png8.

Even if the only thing you do is remove the colors that are very similar, you can easily reduce the number of colors to 18.

Edited by tanuki - 24 March 2011 at 11:28am