Not quite: "blanche" is the feminine form, but milk is masculine ("le lait", not "la"). The masculine form is "blanc", so we need "Le lait est blanc." All vs. some milk isn't the issue -- the gender of milk in French is.
Ex.: Le lait est blanc. ... but: La vache est blanche.
The le/la tells us whether the noun is masculine or feminine. (Honestly, 'un' or 'une' is even more reliable for learning the gender of a noun, but that's a separate issue, and 'le'/'la' makes more sense in this context.)