Yes, REDDER is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, D 2, D 2, E 1, R 1.

Is it a valid word?

REDDER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

of a color at the end of the color spectrum (next to orange); resembling the color of blood or cherries or tomatoes or rubies
characterized by violence or bloodshed
(especially of the face) reddened or suffused with or as if with blood from emotion or exertion

Also: blood-red, cherry-red, ruby-red, red-faced.