Concerning the colour of the Megalodon, you can be quite sure that it was similar to the recent white shark.
If other shark species would have a great range of colours, I would be careful about reconstructing anything. But if you look at the recent species, apart from the ground-living sharks and the giant plankton eaters, most of them have light bellys and darkgrey/-blue backs, as almost all big marine predators have. Or black backs, in a case of orcas and penguins. (Ah yes, giant penguin would be nice!
)
Interestingly, I have added a red shark to my design though, which would be okay if it lived in the deep sea, but not where I put it.