What is physically happening when some marine fish change color?

[chrissreef]

Maybe these are iridophores and leucophores in action or something else? Two examples I'm curious about and what the difference in color changing process is;


1. Acanthurus maculiceps (spotted face tang) can be light gray/brown in general or change to dark with very vibrant yellow spots when excited http://www.dereksreefshop.ca/images/products/TangWhiteFreckled.jpg

and http://s276.photobucket.com/user/CrazyEdie50/media/DSC03314.jpg.html


2. Hoplolatilus chlupatyi - Flashing Tilefish appears iridescent when flashing.


and