[Barrabundy]

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[SharkNett]

Hopefully this covers your question. I know striped marlin light up to make themselves more visible and scary when herding baitfish so maybe your mangrove jack was doing something similar. There are videos on youtube showing striped marlin & sailfish lit up while feeding.

 

[Barrabundy]

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[SharkNett]

Barra, thats pretty much how it works for many species. Others can't do it at all. For some fish species a darker colour is just from sunburn.
Check out some vids of cuttlefish.,I think they do it best of all. The patterns & colours they can make are unbelievable.

 

[ryan]

Mahi mahi or dolphin fish are another pretty vivid example there reel nice looking alive and as they die you can see the color drain out till there silver

 

[Foxxy]

Squid are a bit different - they have things called chromatophores and iridophores. These are controlled by muscles that work like the irises in your eyes, covering or exposing the spot of pigment or iridescence below almost instantly. I'd be interested to find out how fish do it.. I'd guess this is more a hormonal message that exaggerates certain colours already there?