Rolex's rose gold is a proprietary blend that they make up themselves (one of the benefits of manufacturing one's own gold). I want to say that they brand it as "Everose", but I might be remembering it wrong.
I think I saw a movie on Rolex's website that explained Rolex lowered the percentage of copper (which is what was reacting with the chlorine to discolor over time) and increased the percentage of the platinum metals added to the blend. I believe the thought is that the platinum metals "hold" the copper tighter than the chlorine wants to "pull" on it. I'm not a materials guy, so I'm just repeating what I believe to be the case. I looked for the movie, but Rolex's website is hard to navigate. Maybe someone else can find it and we can confirm my memory?