I am entirely willing to believe this but might you have a reference to some grammar explanation? I just reviewed the chapter on adjectives in my bible for such matters, Handbag i dansk grammatik for udlændinge. Good review, but I don't see anything about this most interesting anomaly.
I found my own explanation, p. 40 (online) of Danish: an Essential Grammar: “Nouns used in a general, abstract or collective sense normally require the neuter form of the adjective: Fisk er dyrt. Frugt er sundt.” And, hence, "mælk er hvidt". https://nauchidatski.files.wordpress.com/2013/02/danish-an-essential-grammar.pdf. Xneb knows all! And thanks to our wonderful Danish team.