This is something that probably was thought of long ago, but thinking about it now, not all costumes are as easy to make red or blue without ruining them. So, howabout this: You have a standard deathmatch suit, complete with whatever stuff you want, and then for CTF and other team gametypes, you have a seperate "team" suit, that you have to make to specifically have a certain amount of "colour" to it, with a preview for both the red and blue look. It could be done with a colour "rating" for toning based on the size of whatever object you're adjusting the colour of, and how bright you make it. For example, with a suit, you have a shirt, and undershirt, pants, a tie, shoes, and shades. You need at least 100 colour points for this costume to be usable in a team gametype. The suit is anywhere from 0-100 points depending on how much you let colour influence it's tone. Pants are 0-75. Shoes are 0-20, the tie is 0-10, the undershirt is 0-15, and shades are 0-5. This means you could actually have a black or white shirt provided that you make up for it with the pants, clearly illustrating your team alignment.
I had this idea because sometimes costumes just don't have enough "red" or "blue" on red and blue team, and it makes it really annoying to tell if that person near the flag is on your team or not, and then you feel bad for jumping down and shooting them when they are. This could allow for more variety in colour distribution for costumes in team gametypes, allowing each player to have a clearly distinctive look without allowing players to act as "spies" or just be jackasses that complain when you shoot them in spite of looking like they play for the other team.
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I had similar ideas, but I really couldn't think of a way to do it...
Around here something that isn't very uncommon is people using glow sticks/necklaces to identify teams for night games.
People get like... 3 necklaces or something. And everyone wears them a little differently. So it ends up the glow sticks not only tell you who's on what team, but you can also tell people apart in the dark.