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.
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!
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