So not sure how relevant this topic is, but I've seen people propose the mask became this color due to being exposed to sunlight for extended amount of time, but as it turns out exposing it to pure bleach for an extended amount of time, about 2 or 3 months, has the same effect.
Recently discovered this myself, as I had previously painted my Ruru and was using bleach to remove the paint, as I've done for all my other masks. First mask to have any kind of side-effect from the bleach. Testing it on an orange huna to see if it's perhaps the combination of color and 01 plastic, as I know for a fact the 08 kakama does not have a similar reaction with bleach.
Should note the Ruru was originally orange, specifically from the manas set.