It should be lighter , so when I say lower value I am talking in the greyscale sense in which more value is darker.
I am not certain it should be inverted but when I was reading up on it thats what it said, i guess if inverting it makes it lighter then yes.
the way it works as a general rule is the lower the diffuse setting the more reflective someting is noe in Maya I don't remember if that translates to lighter or darker in the diffuse channel.