I dont know about m90 ( and dont care actually, dont like m 90 at all ) but the red is the color which fade from the sun most of all others,( it is at the very end of the visible spectrum ). My father ( rest in peace) was collecting watches. A lot of diver watches use red on the hour points and it is very common for them to become yellow just after a year of using outside.... It is the same with some cars instruments as well. The wired thing is that the red is fading to perfect yellow without any shades. The spectrum of the sun light and the red color are long known for that. Actually after the redish colors next one which is fading fast is the blue color . So i think if we think about basic physical laws - fading of the m90 is plausible. Dont get me wrong. Im not saying that this is the case with m90 tho.
From other side - if the grayish speakers are faded they must have some issues with the sound too. The material should be slightly thinner already too
And i believe that there will be a lot of not known variants of any boomer popping up around. Some can be faded, modded and so on , but some will be original. Im curious how after 30 years some dj cd player collectors will make some tread about one pair of professionally modded pioneer players of mine. Damn i payed for the mod and cant believe that the color was not factory . All those color scheme combinations is like not good stamped post marks case for me - maybe not the perfect , but rare, interesting and highly hunted .
My M-90 with gray woofers has a bit more punch than my blue ones at the same level with same song!!