[Feticidal]

You can make a mod to change the color of the tracers shot from the MG42 and used in single-player quite easily, but I don't think that's what you mean. I haven't tried it, but I've heard you have to slow down the bullets (via the tik files) and set the tracerfrequency to 1 in order to see tracers in multiplayer. Of course, this would have to be a server-side mod, so unless you run a server, don't worry about it.