white: gheal in a sentence as feminine. geal as a noun. Is this a correct explanation of the use of white?
Kinda but not really in this context. You are right that it would be gheal when attributing a feminine noun, eg. lèine gheal a white shirt. But it would still be geal coming after the tha verb, eg. tha i geal she is white or tha an lèine geal the shirt is white. In both contexts geal is an adjective, not a noun (you cannot use a noun as a predicate of tha).
That’s because the adjectives stay in their base (masculine singular) form when they are predicates of the bi verb (something is white) and only change when they are attributing a noun (white something…).