I think this one should be fairly straightforward. Some sort of "interference" in the pattern has begun between Rand and Mat. As far as I know, Perrin never sees the colors. I would believe that the colors are related to Mat and Rand as Ta'veren. The pattern weaves itself around the Ta'veren, but it also clearly has a plan in mind for them. By passing through the doorway, and getting his amulet and spear, as well as his old memories, Mat might have bent the pattern in a way it wasn't supposed to go. As a result, some action or incident he was supposed to do with Rand as part of the "pattern's plan" may now be fuzzy... the colors may be the pattern fighting to both control and weave itself around Mat at the same time, at least in relation to Rand.