Blonde hair chestnut beard - not uncommon.
I write left handed and do everything else right handed.
When I was young I had blonde hair and a light red beard. I still have all it (although part of it reproduced and migrated to my back) but it's mostly white.

It kinda bothered me as a child at the time, but now my facial hair and head don't match either.
I think its a real rather than fake moustache; a fake would have more closely resembled Redford's hair. He has it in all photos, off set, I've seen of the time, and matches his facial hair in other films, such as Jeremiah Johnstone.

I got red beard and black hair, it happens.

For the same reason you sometimes see a blonde woman with a dark bush.

Well maybe the coloring on your TV was off because though it is a different color it only looks dirty blonde to me. Also, I have dark brown hair with red and blonde in my beard.

Yeah, and why does he shoot left-handed but write right-handed?
He's obviously ambidextrous. He also shoots two guns at the same time, one in each hand.

I was blond when I was a little bambino, now its black although when I grow a beard there's reddish glints in it. As far as left/right, Redford is a lefty, watch some of his other films

I know. I was just adding to this fascinating list of life's mysteries. zzzzzzz

I write like a right handed person but eat like a left handed person. These things happen.

Yeah, and why does he shoot left-handed but write right-handed?
Why? Why?

These things happen, I have brown hair and a ginger moustache.