I'm coloring a picture I drew a while ago in Krita, but the problem is I can't color under the lineart because the lineart and white background of the picture is in one layer, so any color I try to paint on is underneath the white background of that layer.
How can I turn all the white in that picture to transparent so I can color it?

In Krita we usually use the color to alpha filter for this. It's under filters->color.

I wrote a tutorial once that covered all possible ways I could of here: http://theratutorial.tumblr.com/post/66 ... layers-are

But color to alpha is the most superior method.