Not that exact issue, but that is a simple and rather pleasant OCD symptom compared to a lot of the ones that people get and I am sure he has more symptoms than that. OCD manifests itself in many different ways, so it is rare that anyone has the same exact compulsions even though some may overlap. Does it bother him that he can only eat the green ones or is it somehting that bothers you? If he is OK with just eating the green ones, then maybe it isn't an issue, however if it does bother him then maybe you guys could practice eating the other colored ones together. There is probably a negative thought that preceeds the eating of M & M's, so ask him what that thought is and the assure him it isn't true and the prove it to each other. maybe you eat half of a yellow and then he can eat the other half. This is called flooding. It is common method used in conjunction with cognitive behavioral therapy to help sufferers of OCD. I would consider finding a CBT specialist and you and your fiance can work through these together. Meditation also really helps with the random negative thoughts that usually preceed the compulsions that may or may not exist.

Good luck,
OE