Complementary colours to black or yellow would be white and purple/violet, respecively. Or a mix of yellow and black, which would be an olive green. I guess any of those would work without disrupting the paint scheme too much. You could also go for the company colour (idk how closely the scythes follow the codex astartes, however). Also, whatever you use, freehand and patterns might to an extent conceal the base colour... Hope that helps. Oh, and it also helps if you keep cloak and eye lenses (in case he wears a helmet) the same colour.