Translation:Milk is white and chocolate is brown.
Nouns and adjectives after "estas" never take the accusative ending.
Li estas knabo.
Lakto estas blanka.
I wonder the same. Since I'm a Spanish speaker, we generally put definited articles (él, la) before nouns when we talk about general things, like "La leche" to reffer any milk in the world o "El chocolate" to describe any chocolate in the world. But since the point of view of an English native these words don't need articles, unless you are describing a specific milk or chocolate. I guess the logic would be the same in Esperanto, but maybe I'm wrong.