Informally, aircraft with green wheels are ready for operational service .
I must have missed the memo for that, since when did green wheels signify operational service, informally or otherwise?
Because I just checked the Chinese air force's combat aircraft just now and the only aircraft with green wheels are the Q-5 variants, J-7 variants, and flanker variants... And even then not every wheel is green, e.g. J-7s only have the forward wheel green while the aft ones are not, vice versa for flankers.
J-8 variants, JH-7 variants, J-10 variants, JF-17, on the other hand all never have been seen with green wheels either in service or in prototype stage, as far as I am aware.
J-20 doesn't have green wheels for its prototypes either so it is doubtful its wheel colour will somehow change once it enters service.
In other words, I believe there is no evident to say green wheels in Chinese air force fighters represents "operational service" because there are so many types of aircraft in service without green wheels. It would be more accurate to say a few types of aircraft on service (flankers, J-7s and Q-5s) have green wheels.
I also believe there is no evidence to suggest that wheel colour changes between prototypes and in service aircraft. I have not seen such a thing occur for previous aircraft projects and I have no reason to believe it will be the case for J-20.