regrind is usually bright neon-pink or purple though, isn't it? these are red, not neon pink.
They get a couple hours of sun in the morning on the windowsill and then nothing all the rest of the day. And I don't have a car to stick them in either.
I think they want to go rejoin the herd soon.
We tend to think of it as pink because thats the breakdown we see the most - regrind can be any colour, it will be the colour of the original plastic which is agitating. However, the regrind added tends to be for a colour mix close to the body of the pony so for this one bright red would perhaps not make sense.
Unfortunately there are no other solutions if the sunfading is not working you either need to find a way to get them more sunlight - in a car is a perfect idea - or simply let them re join the herd