The X's themselves are, but the overall markers are mostly red (name and arrow). That's a big part of why they stood out.

I'm not sure why you insist that white stands out better than red. Halo 4 doesn't have strong very much in the way of pure, saturated reds on its maps, but it has a ton of whiteness. In any case, a two-colour solution like what Reach uses is probably better than any one-colour solution.