We have played with some darker displays, but the problem is that the spectrum emitted by the display is actually blue-green, so the nearer orange/red you get the dimmer the display appears. We tried a Audi-red display and it looked great.... at night. During the day it was too dim to be useful :(

Unfortunately, blue-green is the standard VFD colour, so there's really not a lot we can do about it.

hugo