Step 1:
Color your drawing. The temptation can be to do the color holds
first because they just look so cool, but it makes a better
cupcake when you put the frosting on last instead of trying to
do it first.

So, I've decided that I want the lines on her shirt to be
colored so that it looks like sheer fabric. I also want to color
some of the lines on her hair because there's too much black.
And just for fun I'll make the hoop she's holding glow. |
| Step 2:
Select everything on the lines layer and copy it
(Keyboard shortcut: Ctrl+A, Ctrl+C). |
| Step 3:
Create a new layer and fill it with black (If you've looked at
the flats tutorials this technique might be looking very
familiar). |
Step 4:
Switch to Quick Mask Mode (keyboard shortcut: Q)
 |
Step 5:
Now that you're in Quick Mask Mode paste in the lines you copied
(Ctrl+V)

Your lines will be colored and you'll see the little
marching ants around the outside. Don't be confused that you
don't have a white border like I do. That's there because I have
a white frame layer over all my layers. It has nothing to do
with the lines. |
Step 6:
Switch back to Standard Mode

The marching ants will now be surrounding your lines |
Step 7:
Hit the Delete key

And all the black will be gone. |
Step 8:
Deselect everything (Ctrl+D) and lock the
lines layer

(yes, I did just reuse and image from another
tutorial) |
Step 9:
Now use the brush tool to add color to your lines layer. Since
it's locked none of the color will be able to get outside your
original lines.
 |