Why is the megalodon that color?

Answer

No one can say with absolute certainty what color the megalodon was. It is assumed that it followed the same pattern as all comparable animals, with a darker back and a paler belly.

Answer

For hunting adaptations. When prey is above the Megalodon, its darkish blue color blends in with the depths and colors of the ocean. When prey is under the Megalodon, the white underbelly blends in the the bright top of the water, where the sun shines down.