[thedoc (OP)]

Fossilised remains have revealed that ancient marine-dwelling dinosaurs and turtles used the pigment melanin for camouflage and keeping warm

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[Professor Mega-Mind]

Not actually dinos , but earlier forms such as archosaurs .  They were also warmblooded . As predators , their colorations would have needed to be within certain parameters .  Diving pursuers would have been overall dark , like otters or seals , with some light color underneath  for ambushing prey from above .  Sharks and birds of prey use this camo. scheme also .  Variations include stripes , patches , & rings .  Black , brown , gray , & white are the basic colors .
 Otay , you well done now !   P.M.

[Professor Mega-Mind]

Exception : Spinosaurus
These would have been a solid tan color , the better to blend into their preferred habitat ; sandy dunes .  N. Africa 100mya was a flooded , riverine environment , with modest vegetation , but sand & dunes everywhere .  The Spinosaurus' were not marine lizards , they lived more like otters , or perhaps , fishing cats .  Like those , they hunted primarily in shallow water , using both shore-born ambush tactics , and water-born lie in wait tactics .  The thin-hump cast a very large shadow , which ( as with crocodiles ) attracted fish , seeking to avoid the tropical sun .  Spino.s long neck & head were very good at suddenly snapping up smaller fish .  Larger fish would have been grabbed midsection  ( to avoid severe thrashing forces ) and hoisted onto land for dissection .  Lifting & carrying large , heavy fish is why Spino.s jaw had a crook in it .  It is also why it's legs were large  ( though this also helped it to ambush & chase prey , especially in river-deep water ) .  Spino.s sandy coloration , and sand-dune imitating hump were actually needed .  They helped camouflage it for ambush , and helped hide it from competing predators as well .  These competitors would attack opportunistically , much as sparrows mob crows . 
Okay , old story over .      P.M.

[Professor Mega-Mind]

Note on Spinosaurus .
These had a smooth surface (think penguin ) , hydrodynamic shape (think otter ) , and smooth & thin-lipped face ( think stretch seal ) .  The eyes were also very seal or dog-like .  The bulge of the eyes is why it needed that protuberance between them .  It helped keep flopping fish fins off of Spino.s eyes .
Alright , 'nuff said .      P.M.