I'm not saying that safelights HAVE TO BE red. I'm just saying that this particular shade of red works pretty well for my application.
(Which is developing regular black and white papers and similar.)

Rosco #R-27 theatrical gel blocks almost everything with a wavelength smaller than 580 nm. and transmission doesn't hit 10% until you get to 620 nm. R-27 transmits only about 4% of the total light. The specs say the stuff knocks off 4.7 stops.

One sheet ought to do it but two sheets sandwiched under frosted glass gives pretty good safelight protection.

I like the idea of the LED lamps. Their bandwidth is so narrow that they are virtually monochromatic like a sodium vapor lamp. The next time I am in the market for safelights I'll definitely be looking into LEDs.

(BTW: Do sodium vapor safelights give anybody else a headache after you've been working in them for a while?)