like choosing color schemes? thats a personal preference but I'd say most people copy what their in-game spartan looks like. unless its a named character then the colors are decided for you. theres a few mash up spartans too that have unique color schemes based off anime characters. stencils get painted after the main color and before weathering. deciding what you want stenciled is totally up to you. find references for accurate in game stencils or paint literally whatever you want on it, like emblems, logos, pin-ups for example.