Yes, REDDISH is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, D 2, D 2, I 1, S 1, H 4.

Is it a valid word?

REDDISH 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

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