This is how I color in oC1 n_n;

1. Start a new layer in oc1, make sure it's set to multiply - this way you can color and the black ink shows up overtop the color anyway.
2. Lay down your flat colors. Set your pen size to however big you need it and feel comfortable, just make sure you have disabled transparency - so the color is bold.
3. To begin shading, start with a darker tone and make sure you enable transparency. Use light strokes. I do it all one one layer because of the multiply setting.
4. Pick a color between your base color and your dark shading color, and go back and blend in everything.
5. Now pick a lighter color and start doing highlights, I generally use 3 different shades lighter for the hair, since it just look sbetter that way :P Touch up where you need it, but this is basically how I color n_n;