Download and install. Lifts up lineart that is bounded to a white background and is locked as a "background" layer to a transparent layer of its own. I didn't make this method, just made the action. The method I found on a tutorial somewhere.
If you have a lineart in gray or pink or whatever, after using this action, it will still be gray. This action works with the value of the lineart, so black will be black, white will be white, gray will be gray, light blue will be gray, dark green will be dark gray. Use some level magic to make it black. After it's on its layer, you can lock transparent pixels, and colour the lineart any colour your heart desires.
If you are using this action on something with lowered opacity, the opacity will remain the same. I don't know how to fix this problem.
I don't make actions, I don't make resources, but people have asked for help twice, and apparently that's enough for me to make this. And hey, if you can direct people here that'd be awesome. Just saying.