This is definitely a tough decision.
It's understandable how you'd want to keep the game's style true to it's mil-sim roots. However, if you do enforce a strict dress code you'll lose a lot of talented teams from playing simply because of the obligations they have with their sponsors.
Some of the best teams out there run around with bright colored jerseys, but in no way represent speedball. I don't think we should be associating speedball gear with the speedball atmosphere. Personally, I'm a hardcore woods/scenarioballer and I wear some gear that's meant for speedball. It's not to look pretty, it's not to look flashy, it's for one reason only: practicality.
To me, a major aspect of being tactical is to utilize your available resources to give you an advantage over your opponent. I'd trade visibility with gear that's meant to sustain hardcore paintball playing anyday. I'd take padding, venting, and durability over a pair of BDU's that offer me nothing more than camouflage that's rarely effective enough to justify its use. And any good player knows, visibility is all about positioning and environmental awareness, not what color jersey you have on. And if most of the action is going to be taking place in a close-quarters environment, then the color of your jersey is going to have very little effect on the outcome.
Like I said, it's a very tough decision. By allowing these teams to play with their jersies, you may lose a little bit in the looks department, but in my opinion, the level of added competition that they would bring to the tournament makes up for this sacrifice.
Personally, I would allow team jerseys, but only as long as the team matches uniformly.