I was about to suggest watercolour, as it has so much transperancy that you can often keep the line and power of your original sketch as well as have colour.

It's a great start and definitely a fun way to go. I love colour! I sort of go back and forth between bold hues and more sombre palettes. I'm really loveing the more moodier tones I am using in my Singularity ladies when I 'oil paint' them as I like that sort of faded dark 19th c feel. yet, when I did/do my animals in chairs series I verge on Pop art colours with some and in some cases the colour story is almost like one of the characters.

Don't be afraid of colour, embrace and you'll become fast friends!