Well, in the BKK places I have visited (just found old notes, must have been >30, but at last 20+ closed by now) this was never ever a prob for me and rather often the boy exceeded in performing what could reasonably be expected from him for the basic tip. Guys who did their job above expectation I would take again and it mostly turned out even better. As it seems from reports most guys now working in the Jowmtien shops are Khmer-over there the request for ´special service´ also did the trick (admittedly quite a few yrs ago by now and not that many times).
This long list of colour codes I remember also to have seen, but it will not work in TH-tipically farang style.
As der Christian again points at- some savvy online shops in Buffalo Bridge (Sapan Kwai/Kwoy for those in the know) have it presented-with decently clothed guys and some details. There seems to be some Chinese sites (likely from pre-covid) that may tell/show more, but only in chinese.
And there is still Thaimassageboy.com though this seems to live on as long as it lasts