Hmmm, I wonder.

It does look very intentional and even. I'm not sure air ingress would explain it away, unless it was moist and even then you'd be looking at a fairly specific chemical mix for that reaction. Some sea water could do it potentially, but we usually see these go grey-green if they go towards a greenish colour rather than the uniform green of this one.

But it also looks a lot like copper, rather than europium, doped lume. Copper was quite common with radium and tritium lumes, but rarer with the non-radioluminescent materials, except at the start when they used it because of its familiarity.

There's some discussion at the moment over which colour is best for visibility when diving. Light is absorbed more quickly at the red end of the spectrum than the blue the deeper you go, so some suppliers are recommending blues now rather than the traditional yellows. Others argue that flourescent yellow provides better visibility because of the fluorescence, irrespective of its longer wavelength than blue light. Green sits between the two, so maybe an early experiment in increasing visibility.

Interesting too that this still fluoresces strongly - normally, patination from moist air ingress would be expected to impair the fluorescence somewhat. In the end, whether this was a factory/supplier experiment with copper-doped lume or unintended patination, it's resulted in a great-looking watch!