Yes, YELLOWER is a Scrabble word!

Letter values in Scrabble US: Y 4, E 1, L 1, L 1, O 1, W 4, E 1, R 1.

Is it a valid word?

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

of the color intermediate between green and orange in the color spectrum; of something resembling the color of an egg yolk
easily frightened
changed to a yellowish color by age
typical of tabloids
cowardly or treacherous
affected by jaundice which causes yellowing of skin etc

Also: lily-livered, white-livered, yellow-bellied, scandalmongering, sensationalistic.