Yes, GREYEST is a Scrabble word!

Letter values in Scrabble US: G 2, R 1, E 1, Y 4, E 1, S 1, T 1.

Is it a valid word?

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

of an achromatic color of any lightness intermediate between the extremes of white and black
showing characteristics of age, especially having grey or white hair
used to signify the Confederate forces in the American Civil War (who wore grey uniforms)
intermediate in character or position

Also: grey-haired, gray-haired, grey-headed, gray-headed, white-haired.