When you devamp a sprite, look for other similar pokemon made by game freak.
Study the way they shade and the colors they use, you will notice many times some more "evil" looking pokemon actually use bright colors, this is because the games were made for Gameboy Color, which had no backlight.
The best advice I can give is practice and study game freak sprites. And don't be afraid to experiment. Remember, two colors, plus black and white. Small Pokemon only get 40x40, medium 48x48, large 56x56. It's okay to bend this rule sometimes but for baby Pokemon you never want to make them 56x56.
Game Boy / GBC - Pokemon Crystal - The Spriters Resource
Game Boy / GBC - Pokemon Gold/Silver - The Spriters Resource
Study the lighting on the sprites too, shading is very important. It's called dithering, and you never want too much or too little.