There is a research document that I do not understand that well, but it says the gills brighten and darken in colour due to cyanide poisoning. The link is below... I don't know if this applies to aquarium fish since these fish were exposed in a lab, but I do believe it also speaks of fish that died in the rivers. The tests were also done on salt water fish as well...
http://www.google.ca/url?sa=t&source...q4HXoF-8932drg