The bottom level of RGB is the consensus layer, which is designed as an immutable protocol targeted for consensus level ossification.
Before the ossification happens all changes to RGB must be backward-compatible; such changes for client-side-validation are named fast-forwards. Development and deployment of fast-forward changes to the consensus level is performed by LNP/BP Standards Association.
Consensus level includes a set of RGB-related standards, also maintained by the LNP/BP Standards Association, named LNPBPs. They can be found in the standards repository, which contains also description for the standardization process and acceptance of the standard proofs & improvements.
It is strictly not recommended to use any alternative implementations of the RGB consensus level.