Change Revel BE driver color to black?

I am assuming you want to paint them?

Not possible without an audible effect. Whether your ears can detect the change is another story altogether.

I don’t know about Revel, but the top speaker manufacturers weigh their cones and all other moving parts down to the milligrams and carefully chose the cone materials, mass and thickness, as well as many other parameters to perform as they want them to. Paining, staining or any other modifications will definitely alter the sound and performance.

Those don't come with grills?  If not, a better option would be to custom make some yourself or have them made.  Personally I was/got over wanting to see my drivers long ago.  If it was up to me speakers would be invisible!

Wait for nighttime, then turn off the lights - plenty of black :)

Why would you want to do that?

Impossible, and a ridiculous idea!!!

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.