Where does Chocklate Milk comes from

Today when Joshua drank milk, he said in Chinese: "I love cow milk, it tastes really good!" Then he asked me why cows produce milk? I said because those cows (black and white cow) are especially cultivated to produce a lot of milk for us daily. Then he said, "Oh, so those brown cows will produce Chocklate milk, right?" I startled. Why I didn't think of it? :-)