it's Anti-aliasing. Behaves differently depending on the background color and pixel alignement.

Red over green creates particularly harsh contrast due to how RGB screens work. This effect occur between all the 3 primary colors of light red, green, and blue, but it is most noticeable between red and green.

The screen has to blend them on edge by mixing them, but since they share no common subpixels, this creates a dark and muddy color (brownish, yellowish, blackish).

I first noticed this specific combination when I was drawing my country's flag —Morocco. XD