Just reference real life, judging by your picture and the harsh shading it seems like his skin and clothing are all the same. Soft shading for skin and generally darker shading on clothing etc.
Also emphasize the shading on the clothing too so you add that distinct difference. Its good to add some variations of shading too, just every once in a while.
It really depends on the look you want. Referencing really helps though, from life and other artists.
Anyway theres my shit talkin' over.