Photoshop: Help needed

I'm trying to find out:
How do you color "under" the lines of a BW line drawing? Drawing in a layer covers the lines.
And while I'm at it: how do you alter the gradients so that the go "behind" the background layer?
Help, please!
How do you color "under" the lines of a BW line drawing? Drawing in a layer covers the lines.
And while I'm at it: how do you alter the gradients so that the go "behind" the background layer?
Help, please!