The file you need to change is in the Sparks-gfx (C:\Soldat\Sparks-gfx by default) folder, and is called "odprysk.bmp", a one pixel bitmap that you can use to mod the /piss particle. Unfortunately, this also changes the sparks that are thrown off by explosions and ricochets, and will always have a yellow tinge (for example, you make odprysk.bmp a bright blue, and the /piss particles will be a very bright lime).