The color of the rubber has nothing to do with its compatibility with different refrigerants.
Toyota's R-12 o-rings are brown, and Toyota's R-134a o-rings are black. I've never seen any green A/C o-rings from the factory on any Toyota. Toyota's TSB for A/C retrofits say nothing about changing o-rings for the Celica, so I'm lead to believe that they are made of rubber compounds compatible with R-134a, regardless of color. You can't just throw Toyota's R-134a o-rings on the R-12 lines, they're completely different sizes. I'm sure someone out there makes aftermarket o-rings that fit as well, but I've always used the Toyota ones. I put new Toyota o-rings on my all-trac with R-134a in it several years ago, and they've been holding strong for several years now. My sniffer can detect leaks as small as 0.1oz of refrigerant per year, and I can't detect any leaks. My other Celica had the A/C system refurbished several years ago with a new compressor, hoses, o-rings, and new old stock R-12. It's also holding strong.