Magenta, Salmon, Green and Orange will all fade pretty much all the way to white. When I was a kid I used to play with my MUSCLES in the sandbox and there were some figures that got left out there for a couple of years in the sand and the side that was facing up toward the sun got completely whitewashed and the parts that were submerged in sand are not faded. The Blues, Purple and Red don't have nearly as much of an issue with fading but I've found that they sometimes get discolored. I wonder if the discoloration has more to do with them coming into contact with some substance that reacts with the plastic or the dye than it does from fading? For instance, if Blue figures were to be washed in Bleach or Rubbing Alcohol if over time the reaction would begin to change their color? I'm not a chemist and I have no idea how these things actually interact. Also worth noting is that I didn't have badly faded Blue, Red, or Purple figures from my childhood sandbox. So, its possible that there simply were not figures of those colors left in the sandbox but it's also possible that there were figures of those colors left out for a couple of years but that they didn't fade like the lighter colors did.
Another thing I've noticed is that purple figures sometimes get this filmy white coating on them that is (so far as I can tell) impossible to wash off. It's like the purple turns to a scabby white on the surface but it's not from fading it appears more like some kind of a reaction between the surface of the miniature and the air. I've encountered like four different copies of this Sunshine figures (all from different lots) and am yet to encounter even one that doesn't have some degree of this white streak across the chest and lower abdomen area and on the back:
I haven't actually figures a way to get it off the surface of the figure yet which is what sort of led me down the road of thinking that the discoloration on the figures that don't tend to fade like the lighter ones might be more of a reaction than straight sun fading. I'm by no means an expert on this stuff but these are just some observations I've made over the past couple of years collecting MUSCLE figures. Cheers, Brian.