So, buttons with the properties of milk plex would be cool, but the downside would be that they would look white, even when not lit up.

This is exactly what I'm talking about: Very high diffusion, low opacity.

I think it would be coolest to have milkplex-like buttons, but have the milky color be gray like the original buttons. Then you would get the same thing: high diffusion, low opacity, but have the color of the glow be a dimmer shade of gray instead of white. Heck, that's what you'd want anyway, having the buttons glow only dimly instead of brightly.