Pied feathers in green cheek conures are typically yellow, not red. It's also normal for some of them to have red, orange or yellow feathers at the elbow or "bend of the wing", especially in the opaline (aka yellowside) mutations.
Copperarabian, unless the grey was born with those red feathers, I would guess the red feathers are due to plucking, not a mutation. Damaged feather follicles can (not always) result in red feathers as well. I've seen a few photos of plucked african greys with red feathers, and even the famous Alex (Irene Pepperberg) had a red feather, and he wasn't born with it. He had also plucked on at least two occasions.