Hi John
These people have done their homework regarding matching bike colours covering many years, they now want to get a return on this work by selling paint and who can blame them?. They are not about to tell everyone which paint code they need, as you will have seen on the RS Paints website they do charge top price.
Some specialist marque parts suppliers can give paint codes and some owners clubs also have colour lists, but yes it can be a problem.
I have always found on bikes I have restored, if you keep asking people someone will eventually know a colour match.
Sorry but I don't know of one source for the info, I guess if you are stuck you just have to pay the RS price, lets be honest the cost of the small amount of paint needed compared to the total restoration cost wont break the bank.
Regards
Dennis