that’s not a dumb question! there are a couple of ways to do it, but i’ll show you the two that i usually use
the method i use most often is to take my line layer
and lock the transparency, as such:
all you need to do after that is just take the paintbrush (or your tool of choice) and draw over the area you want the color in. because the transparency of the layer is locked, the color can only go on the opaque parts of the layer, which in this case is my line art.
TIME OUT!!!! the drawing you linked me to as an example (http://dredsina.tumblr.com/post/39772819254) started as a pencil drawing that i scanned. if i were to attempt the previous technique with my scanned drawing, it would turn out like this:
because there is no transparency in my scanned layer! it is a solid image. the technique wouldn’t work. to make it work, i would need to turn my scanned layer into a transparent line art layer, which is what i did for this particular drawing. how to do this is to go to your “channels” tab:
click “load channel as selection”
then, making sure the tool you are using is a selection tool (marquee, lasso, or magic wand), right-click the work area and click “select inverse” (if you are not using a selection tool, this particular menu will not show up when you right-click)
now create a new layer!
then, using the paint bucket tool, fill the area with black (or whatever color you want the lines to be). make sure your paint bucket options are as such:
you now have a layer where everything “white” is actually transparent — an isolated line art layer!! cool!
there’s another option i use occasionally, though this is actually a technique i usually use for flat colors.
how it goes is that i make a new layer above my line art layer
draw the color i want in the area i want it to be
right-click the color layer and select “create clipping mask”
and that will make it so the contents of the selected layer will only affect the opaque (and translucent) elements of the layer below it.
et voilà! sorry this post was so long, but i hope it helped!NOTES: 532