Step 1: Hold down C go to player model at the top left of your screen
Step 2: Left-click on color
Step 3: Pick a color you want to have on your physgun

you can mix up the colors to something you like

my fav:
aqua I0ve
gives the gun a great aqua bIue, green c0I0ur

Red: 100
Green: 901
Blue: 400

i know this is something everyone knows but i thought be might be helpful to some people :)