It's probably indirect UV light from the sun. I have a Macross VF-1, white with red trim, and even though it is not exposed to direct sunlight, the decals on the "front side" in the glass cabinet where it has been for 20 years have faded considerably - also on another Macross mecha (the red in the "kite" roundel), but there it is less obvious.
However, there are real world cases with similar weathering/bleaching which look
so weird/strong that noone would try this on a model because it looks "unrealistic".
