materials can effect how an entity is lite ( thats what materials and shaders do afterall, effect pixel color intensity)

since most the materials i've seen use some form of ambient light setting influance. adjusting the entitys overall ambient setting I think would be overriden by the materials effects.

only solutions i can think of alter your overall level lighting intensityes ( sun/level ambiance to get overall more or less lighting influnace

or alter the models skin, giving it more or less contrast/brightness