Change Revel BE driver color to black?

Beryllium is an expensive, lightweight, very hard material that's great for tweeters.  I’d expect paint to add mass and change the sound, sort of defeating the purpose of choosing BE over other materials.  Don't do it.

I can’t image that would be possible without big effect on sound… negative.

Again, why?

If it bothers you that much, perhaps a different speaker could be in your future.  If you modify them, they will be worthless if you try to sell or trade in.  In other words, do not do it!

The weight of a berillium tweeter diagram is on the order of 5 to 10 miligrams. A mid-sized duck feather is 200 mg. Let’s spray a light coat of paint on that… what double the mass? I don’t know but it will be a non trivial weight change.