Yes, COLDEST is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, L 1, D 2, E 1, S 1, T 1.

Is it a valid word?

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

having a low or inadequate temperature or feeling a sensation of coldness or having been made cold by e.g. ice or refrigeration
extended meanings; especially of psychological coldness; without human warmth or emotion
having lost freshness through passage of time
(color) giving no sensation of warmth
marked by errorless familiarity
lacking originality or spontaneity; no longer new
so intense as to be almost uncontrollable
sexually unresponsive
without compunction or human feeling
feeling or showing no enthusiasm
unconscious from a blow or shock or intoxication
of a seeker; far from the object sought
lacking the warmth of life

Also: moth-eaten, cold-blooded.