Because slippy doesnt have a rail, you can just open up your slipgate.cfg and put color1 "x" and color2 "x" in, depending on what color you want your torso/trousers to be (yes, i did say trousers, leave it out
).
Then open your strogg and arena configs, and set your railcolors to what you want them to be
That'll change the colors depending on your class every time you switch classes.
Of course, if you want all your classes rail/skin colors to be the same, just leave it as it is