as far as i know its ok for width, but you need to cut it a little inside the corners to allow it to bend to fit the bumper. to fit it you could either use self-tappers, or drill thru the bottom of the bumper and pop-rivet it.
i spose it wud look better colour-coded if the rest of your car was, but mine isnt so when i put a splitter on im leaving it black plastic. got to bear in mind it could easily become a parking/speed hump casualty so seems better to leave it unfinished to save on paint repairs, IMO anyway.
Liam