@Qi Thanks for the message. I sympathize with what you are saying about color selection. It's tricky. And it's one of the tougher areas of illustration to teach (in my opinion). The main advice I give to students is to pick a color to key the whole illustration off of. Such as "What is the MAIN color cast of this image?" Then all other colors are selected around that main color. Otherwise the palette starts to get out of control for me. But I typically prefer analogous color schemes and color that isn't too saturated. Someone else might prefer much brighter colors or whatever, so it's hard to have a solid rule there.
The other advice I give is find a bunch of images that you like and look at how they are using color. Then steal their color palette! Color is not copywriteable! You can use any of it from other artists. You can even sample their work if you are working digitally. Now isn't this a bad/unethical practice? Nope! The reason that it's ok is that your drawing and value are what really hold an image together. Even if you color pick from another artist, it won't look exactly like they did it because the values and drawing are so different.