Are you talking about paint on the helmet or on the X-wing starfighter itself? For helmets, the paint jobs were custom to each pilot. There were various symbols that some helmets shared, like the "sliced onion" and the red and blue circles with black arrows, but they have no canon meaning.
As for the starfighters, the colors were the colors of the squadron-- red marks for Red Squadron, blue marks for Blue Squadron, etc. Red Squadron (and presumably other X-wing squadrons) had stripes on the wings, and each X-wing had a different number, but I don't think it's even been said what those stripes mean, though it's most likely their call sign. Luke's X-wing had five stripes on each wing and his call sign in A New Hope was Red Five...
