Step 4: Get to colorin'!
I suggest using a color not so bold at first, something that might be close to street color but still distinguishable. That way you can take the extra step of stepping back and checking the accuracy of your shape before adding in all your painful detailing.
ps- I used whatever I had on me (some old color oil pastels) and had the boy pick up a few chalk and charcoal pastels just so we could play with some options. I strongly encourage experimentation;)
pps- Though I used the palm of my hand to spread the color, it'll quickly dilute the color. So for base colors-cool, for detail you want to pop- not cool.