powder blue vs powdery-blue

powder blue is a noun but powdery-blue is not a noun.

Word	Noun	Adjective	Verb	Adverb
powder blue	Yes	Yes	No	No
powdery-blue	No	Yes	No	No

As adjectives, powder blue and powdery-blue are synonyms defined as:

powder blue and powdery-blue: of a moderate to pale blue or purplish blue

powder blue (noun)	powdery-blue (noun)
a pale blue color with grey in it

powder blue (adjective)	powdery-blue (adjective)
of a moderate to pale blue or purplish blue	of a moderate to pale blue or purplish blue