Gary, Bill's explanation about the blue wavelength causing the paint to fluoresce the most, caused me to remember the crystal clear [not opaque] light green filters I found on the F600 dash. Remember, I brought that dash last year? The F600 gauges have orange painted needles, too, so why did Ford go with green filters?
By the way, when you set up your test for different bulbs as well as different paints, if you want to test a green filter, I can mail you one [or all].