R-134 rings are green and can be used with either refrigerant. R-12 rings are black and can only be used with R-12. As far as I know, black o-rings aren't made anymore.
The o-ring acts as the seal, or gasket, in the fitting. Tightening the fitting does not improve the seal.
Flare fittings depend on 2 metal surfaces mating to make the seal, therefore the connection has to be tight. This makes it more prone to leakage.
R-134a works on higher pressures, and the molecule is smaller, therefore the seal must be better than R-12. ALL R-134a fitting are made for o-rings.