I think phosphor quality or rate of wear also has to do with the 'greening effect'. I have fired up many a coated MV for the first time (/DX or /C) and will be astoundingly bright red. Even at full run up, extra amount of warmer color, but after a few hundred hours....while still technically new, the lamps lose that extra bit of brilliance as the phosphors start to burn in from the heat and UV intensity.
That degradation is not that much the phosphor, but dominant is really the UV getting stopped first on the worn out arctube wall. That causes color shift towards colder tones.
Later with the wear becoming more extensive, the blue get blocked as well.
And what remains is then the mercury green and yellow...
This degradation is normal wear for all MV's, with the types with the special mix causing the coat being grey instead of black the dark blackening is shifted later towards the end of the rated life, but still then it happens anyway (when the substance gets consumed) and usually way faster (the use of such whitening agent usually means the arctube is designed for higher loading, to boost the efficacy during the rated life, so the overall life is maintained around the 24khours)...
