If you search on "1935 Temperature Gauge" here on the Ford Barn you will see a number of threads on rebuilding these closed gauges. Here is a reply from one from Morris Gauge that says the fluid he uses to rebuild with doesn't fade:
"I believe Classic and Exotics only sells fluid for hydrostatic fuel gauges. As for temperature gauges, our dye has yet to fade. "
www.morrisgauge.com
There are other people listed who also rebuild these in some of these threads, but this response specifically addresses the fading color issue you posted about.