For the words grå and blå, both the forms grå/blå and gråa/blåa work as definite and plural forms.
Notice they're not interchangeable everywhere, only as definite and plural. I'll show a table so it gets really clear:
en grå bil 'a gray car'
den grå/gråa bilen 'the gray car'
grå/gråa bilar 'gray cars'
de grå/gråa bilarna 'the gray cars'
ett grått hus 'a gray house'
det grå/gråa huset 'the gray house'
grå/gråa hus 'gray houses'
de grå/gråa husen 'the gray houses'
en bil är grå 'a car is gray'
bilen är grå 'the car is gray'
ett hus är grått 'a house is gray'
huset är grått 'the house is gray'
några bilar/hus är grå/gråa 'some cars/houses are gray'
bilarna/husen är grå/gråa 'the cars/houses are gray'
OK — so if I read you correctly, the sentence must require the adjective to be both definite and plural, for the two to be interchangeable.
And, AFAICT from your table, there is no situation in which gråa or blåa is required — it always either is optional or requires the extra "a" to be absent. Is that right?
Also, is this true of any other adjectives like 'små' that end similarly? (Are there any others?)