I used to use gray-scale because i am color blind (not litterally). Also in gray-scale you have a lot shades without making your own.
As for use of gray-scale I have few rules. Generaly you have to decide general shade for whole picture and color every large obiect in shades around one you choose (5 – 6 darker and brighter). Other shades may and even should be use for small elements to emphasize details. It is also important to make contrast between all adjacent objects especially body and cloths . Shadows on body and cloths usually shoud not be to much darker them rest of obiect. I also avoid using six darkest colors because they make lines difficult to see.
Ooh, this is helpful! Next time I try using greyscale I’ll try it.