| The arctube getting gradually dirty on the inside from the material sputtered/evaporated from the electrodes and condensing/settling on the quartz surface. Because it is a kind of dust in nature, as it is building up, it first absorbs the shorter wavelengths, so the UV, then as it gets denser it starts absorbing the longer wavelengths as well.
Because the UV is the component exciting the orange glowing phosphor on the outer, this orange part di inishes as first from the emited spectrum, so the lamp shifts to colder CCT.
Then as the wear progresses, it starts to absorb the blue lines emited from the Hg discharge.
So what remains are the yellow and green Hg lines, so forming the greenish light so typical for the past-EOL high pressure mercury lamp.
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