All females, and even pale males will have that eventually. It's just the color of their gills showing through their scales that have a lack of pigment. As long a she's acting and behaving normal, and not seeming clamped up, she should be fine.
Also, about the food, those foods usually have a high protein content, which makes all their colors, not just red, brighten up (or darken). Unless you got some cheap-o brand that actually uses a high dye content to actually dye the fish from the inside out.