Now that comics are colored on the computer, you can use unlimited array of colors. However, just because you can, doesn't mean you should. With computer coloring there's a strong temptation to over render your comics, and therefore mute your colors.
This is why the comics in general look so much darker and more somber looking than comic books of the past. Now, if this is your intension that's fine, but if not, here's a few rules that will help you keep your colors popping off the page.
1. Adding white or black to a color instantly grey's it down. When sculpting a face with color, try using a color (like orange), instead of black. When looking for a highlight, try a warm yellow instead of white.
2. Know your color wheel and it's complimentary colors. Keep in mind when you add two complimentary colors they neutralize each other and you get grey. So if you are sculpting a yellow object, you should try to use orange-yellow or yellow green. It's OK to use the complimentary color for the shadows, but on the lighted side using the complimentary color with mute your color.
3. Think color into your character ahead of time. If your character is going to be set in a cool colored environment, you might want to give them a warm colored costume (this was the case with my Orange Peel 3 character), to contrast it against the backgrounds. If your character exists in a warm colored place like a desert. you could choose a cool colored costume. Of course you may not want the character to contrast against the background. The point is, think about what you want the color to do beforehand.
Here's two examples of effects that show what I'm talkng about in these rules.
First is a typical modern computer colored comic. Squint your eyes at this and it looks like a mishmash of confusion. I didn't even notice the Loki right away. Everything is muted and blends together. It's hard to make out the Goblin's features from the character behind him. Spider-man is still visible because he's a warm red against the cool background colors. Although this piece is skillfully rendered, it would disappear on the shelf with all the other muted comics. Although there is some warm and cool color separation, there's still too much white and black added to the colors and it grey's them.
Here's another example of coloring using an old comic book. Try squinting at this one. You'll see the figures are still plainly visible. There is no white or black added to the colors, so they scream off the page.
This doesn't necessarily mean one comic is colored better than the other, it's just knowing what you want your comic to do on the shelf, and how to handle the colors to get your desired effect. Do you want to have your colors pop or not, or do you want something in-between?
Remember there's a ton of comics out their and you have to do something that will get yours noticed. The best plan is to go to a comic book shop or convention, and see what catches your eye, analyze why it does, and then incorporate those elements into your own art.