Good picture.
It's well-known that the red lens is the least readable of the options, and that the blue and green lenses are the most readable. It's just a question of frequencies. You're absolutely correct about the blue lens working best because of the color of the VFD itself. This has been specifically discussed before.
You're welcome to try different colors of gels and see what you can come up with on your own. If you can come up with something, there are some folks here on the BBS who have access to machines which can custom-mill plexiglass sheets. If you can find a particular combination of colors that works well, you might be able to talk them into milling some faceplates for you.
Another option would be to use a clear piece of plexiglass to which you attach a piece of acetate that's been printed on an inkjet printer. There are some folks on the BBS who have done this sort of thing. They make acetate that's designed to be printed in inkjet printers and then used on an overhead projector for boardroom presentations. You could experiment with different colors on your inkjet printer and see how they come out.
Remember that with any hand-made faceplate option, you're giving up FCC compliance as the back side of the faceplate has been treated with a electrically-conductive coating to block RF emissions. A home-made faceplate won't have this coating.
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Tony Fabris