This has been a good test since the C&TS left the green on the engine for the whole season, or at least long enough for it to get good and dirty. So much ink (electrons?) has been spent on "did they or didn't they" and trying to prove one way or another with black and white photos and we now have proof positive that a green boiler looks like a gray-black boiler after it has been in use for a while.
I think we just have to accept that paint card as the real deal and say that we haven't seen much positive proof of a green boiler because you can't tell if it is green or black with a black and white photo! Here's the above in grayscale: