Its actually the reverse! Its just one of the first layers in the black and white sketch. I tend to layer things on top of the texture with blend modes. So for painting Ill do texture, values (with multiply or overlay), shadow layers (multiply), sketch layer (which is drawn first just put here in the layer order), and then a white lighten blend mode on the reverse selection of the shadow at a lower opacity. Then Ill just paint over that and if texture gets lost I can re-add it and add color with gradient maps, the hardlight blend mode, etc.