First you do with the B&W lineart is to go to the channels. Then press Ctrl+left click in any channel, inorder to automatically select all the white area. Afterwards you inverse the selection, so that the black lines will be selected. Then you delete that layer and make a new layer of which you fill the blank selection with black. Then I cut each line in a different layer, then Ctrl+left click that new layer inorder to select the lines and fill it with the desired color.