In Photoshop you can use le level to make some gray tones in white. Then you can select by color the white and delete it. When you have only your lines you can put this layer on multiply and on others layers you can paint what you want ^^. (I'm sorry for my poor english)
I would offer you to try Paint Tool SAI. It has a great lineart tool which I often use myself,even though I sketch digitally. Unlike MS Paint line tool, it's much more flexible and easy to use, you can also adjust pressure there, edit it without ruining all the picture, and etc. SAI is generally one great program, really.