Why is vit not proper in this context?
Because salt is an ett-word and then the adjective has to take a -t.
What is the difference between vit, vitt, and vita?
Vit is used for en words, vitt is used for ett words like ett äpple and ett salt. Vita is used for the plural of object like vita skor (white shoes)
So the adjectives must match the subject even for collective nouns? (I don't know how it is called, but I mean when the subject is used without any article or definite form like salt in this sentence)
Adjectives modify nouns. It must match the noun it modifies. In this case it modifies the noun that happens to be the subject.