If you're a kid, your natural curiosity must have triggered this question: Why is milk white in colour? And if you are a harried parent who's already fielded questions on why the sky is blue and why the grass is green, sooner or later you're going to have this one thrown at you.
The protein contained in milk is called 'Casein'. This protein is rich in Calcium and is white in colour. Milk also contains fat, which adds to the whiteness. That's why low-fat milk is less white in appearance (though you may not be able to spot the difference with the naked eye).
White objects do not absorb much light - that means most of the light gets reflected. A tomato looks red because it reflects red light and absorbs the other colours of the spectrum. Milk looks white because both Casein and cream (fat) reflect light.