RGB Specifications
Black paper
Unlike an ICO "whitepaper", the RGB blackpaper systematically explains RGB, its capabilities, applications and most importantly, does not shill any tokens.
Yellow paper
Yellow paper gives a formal description of RGB system in terms of distributed computing and computer science. It is a work-in-progress and will be released later this year.
LNP/BP Standards
RGB is also defined in a number of LNPBP standards from LNP/BP Standards Association.