The animal that has males tend to color up and turn a dark blue during breeding with a blue dorsal fin (located on the back) is the Nimbochromis livingstonii
The dorsal fin may also show blue coloration while maintaining an orange to red band and sometimes a white line. Adult males frequently "color up" in response to changing conditions and rapidly change from spotted camouflage to brilliant blues and greens and may even exhibit a pale golden tone. Breeding males turn a dark blue which almost completely obscures their blotched pattern. A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. 
