This implies that it is NOT possible (see second post). This says they do not cross breed in the wild, but that is kind of vague . . . and doesn’t answer the question of viable offspring if they were breed in captivity. Finally, this implies it might be possible . . .

I think you saw a variation or mutation and not a cross-breed.

Here’s a taxidermy of a red/grey mix

I saw two with gray bodies and a red face the other evening. I figured they were juveniles but do not know for sure. Did yours look like these two ? (Single click any picture and it will come up larger.)