I just copied this directly off the USPTO website referring to utility patents. You might want to check with them.
Hope this helps.
11.13 Special Requirements for Drawings
(a) Drawings shall be executed in durable, black, sufficiently dense and dark, uniformly thick and well-defined, lines and strokes without colorings.
(b) Cross-sections shall be indicated by oblique hatching which should not impede the clear reading of the reference signs and leading lines.