i scan my art type drawings in full color just because it will pick up better detail but if it is a greyscale sketch use grey scal

black and white is usualy better for documents or inked patterns

also what type of resolution are you using?  high details needs a higher resolution

a simple line drawing can work well with 72PPI but a full scetch with any shading needs more i scan at 2200-2400 but i mainly scan technical drawings that need full detail