Playing with Fades

[Drake Raider]

Hey yo. Idea struck me yesterday. Typically Fades, used for fog and the like, replace the darker colors in the map, and disable Fullbright sprites.

Is this calculated using color of pixel or actual brightness and distance?

The reason I ask is I was thinking it would be cool if I could use fades to tint other colors, or to leave brights as is. Having multiple fades a ma. In mapinfo would also be cool.

I don't know if this exists already, might need to be a feature request, but if it is possible, how would I do this?

EDIT:
I should probably start by asking how map defined fades interact with sector ones, if they override or blend? I'm hoping the latter.