I think the X-wings were originally intended to be a totally different color as well. In the novel-based on a slightly earlier version of the script-the X-wings are "Blue Squadron" but this was changed as blue probably didn't photograph well against well, a blue screen. So they were given red markings instead.

Both The sequel trilogy and Rogue One have Blue X-wings though (The Rogue One squadron was mostly destroyed though), maybe a slight nod there.