I will concede that it is necessary, to allow grey monsters to offer some threat, if you allow red PCs to PL lower level players. It keeps the play from being totally trivialized. I do think we should have a good chance to resist some of their CC though, like they enjoy 100% resistance to ours.
That being said, I think it would be nice if you gave players a little more effect on red monsters for a few levels more.
The level range of make shift parties is generally not as tight, as your current con system seems to want groups focused. It isn't uncommon to have 5 levels of gap between party members, and shouldn't be that big a stretch, if they are tackling something white to the highest level player. DAoC had a system where everyone's hit chance was elevated to the highest level player in the party, to enable more grouping. That said, you can still get creamed when a red hits you. Just have to be very careful. The current dynamic, just makes the low level person in the group, relegated to torch bearer, which is no fun. Or forces the highest in the party to stick to greens/low blues, which aint as fun, as getting some progress on the menu.