I have previously used white CT1 when painting back to wall concealed cistern loos. I wanted something that I can paint up to and do so again x amount of years down the line. I painted the back panels before applying the CT1. It yellowed.
I think it may be something to do with a lack of UV light. I suspect that all of the MS Polymers act in the same way.
MS polymers are great products but the manufacturers are often far from honest about the limitations, for example, CT1 says it can be over painted but they don't tell you that you have to lock it with waterbased paint first. I once made the mistake of using some to fill some gaps. Two year later, I forgot it was there and used oil based eggshell over it, the paint was tacky for days. I had to hit it with waterbased primer and then repaint.
And, I have seen the clear Ct1 yellow as well. The dark colours such as black and anthracite seem to be more stable.
