Colour can be tricky. The most important thing to know is that even when using colours, you need to know how tones work. Different colours have different values, which you will be able to see when you convert your drawing/picture/photo to black and white. That's the first rule described by Melanie Gillman below. Check out the handout she made for a colouring workshop. It's a great starting point!
Melanie's website: http://pigeonbits.tumblr.com/post/8672102194/a-handout-i-made-for-a-coloring-workshop-im