New research, mentioned in a post on National Geographic, suggests that great white sharks can change colour—perhaps as a camouflage strategy to sneak up on prey.

In an experiment off South Africa, researchers dragged a seal decoy behind a boat to entice several sharks to leap out of the water close by a specially designed colour board with white, grey, and black panels akin to a photographer's grey card.  

Each time a shark jumped out the water it was photographed and by using the colour cards as reference the photographs were corrected to account for variables such as weather, light levels, and camera settings

It was then noted that one particular shark appeared as both dark grey and much lighter grey at different times of the day.

Next, when analysing small skin samples under a microscope, the researchers were able to observe and video-record how the great white’s melanocytes—skin cells that contain pigment—contracted and turned lighter in colour when doused in adrenaline.

At the same time, another hormone, melanocyte-stimulating hormone (MSH), caused the same cells to disperse, resulting in darker skin colour.

With data from a limited number of sharks, the scientists caution that the great white’s ability to alter its appearance is not yet validated and that their research has not been published in a scientific journal. But other experts say the possibilities are tantalizing.

The leading hypothesis for such changes is that the sharks were getting tans after spending more time in shallow water where the sun’s rays are stronger.