You show up at a field in shorts, and everyone will be aiming at your kneecaps.
Also, it's incredibly discourteous, to both the host and the other player, to show up at a game that blatantly stated the uniform requirements not wearing any of them. It's annoying for host who now has to accommodate someone who just basically gave them a big middle finger and doesn't fit into their team separations, and it's annoying for the players who wanted some semblance of uniformity among the teams and now they have an odd dude out. Some games are more lax than others, but say 'fuck you' to the host enough and you'll very quickly find yourself without games to play at.
Airsoft isn't just about you having fun -a good airsoft player considers how his attitude and deportment effects the rest of the players as well.
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."