"Platinum OA" basically means that the publication is openly available to readers from the publisher's site (just as in Gold OA) and that no fees have to be paid on the side of the authors (just as in "Green OA").
Sometimes, "Diamond OA" has been used for the same concept, but this may cause confusion in some who may think that there's a fundamental difference between "Gold OA" and "Diamond OA", just like Gold and diamonds are different materials. "Platinum" avoids that by sticking to metals and implying that "Platinum OA" is more precious, more desirable or otherwise better than "Gold OA".
Another term that is sometimes used is "Universal Access", or "Universal OA", but this again may lead to confusion in terms of what the universality is referring to.