If you are asking: "Is degradation safe?" The answer is no! If you are asking: " Is temporary degradation safe enough?" Then the answer is maybe. More precisely is the compromise acceptable?
In your example:
Imagine you approach an airport where every green light has been replaced with a white light? Same goes for green lights/displays in the cockpit - they are now white. No other colour is affected. What real effect does this have? Is it safe?
The temporary degradation may be acceptable if other safety factors provide a compensatory element.
In the real world example I gave you, removing the safety discrimination of the green taxiway lighting and substituting it with white now gives you the same colour factor for the taxiway lighting, the runway lighting and the tail lights of the aircraft in front (whose rotating beacon and the safety colour you are now relying on) might not themselves be serviceable.
You could make the same argument for having two pilots. Is it safe to operate with just one? No! Is it an acceptable compromise to operate with one when the other becomes incapacitated? Yes. The compromise is acceptable (and indeed vital) under those conditions of degradation.
Visual acuity is an important tool. Is it acceptable to permit deviance from the highest standard? Obviously it is, since such deviances are commonplace with the standards set at correctable limits. Colour deficiencies do not appear to be correctable at this time, and it would therefore require a compromise to make such deficiencies acceptable. Some countries already do this, and many don't. Is that safe?
Those countries that have accepted the compromise think so. That neither compels other countries to follow suit, nor does it prove that a deviance from the standard is anything other than a compromise.