Coz milk contains 5% of lactose.and also milk is made up of casein protein which is very rich in calcium, and also the white colour fat present in the milk gives milk a white colour.  and these fats give bovine juice.and another reason is that milk reflects all the wavelengths of light but wont absorb any thus its white in colour. and it depends upon the food that cow eats.

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Milk consists of about 5 percent lactose, 3.7 percent fats and 3.5 percent proteins. Calcium-rich casein is by far the most common protein, and it is the combination of casein and certain fats that give its colour. :)