African Butterfly Fish Curiosity

[Milburn95]

Hey, I’ve had my African butterfly fish now for a year and one of them has changed hugely in colour, I've researched them and can’t seem to get an explanation so wondering if any of you knowledgable folk would know, attached is a photo of them side by side for comparison. What do you guys think? 

[gardenman]

The lovely folks at TFH (Tropical Fish hobbyist Magazine) say that colors vary wildly and don't reflect the sex of the fish. Males have a long anal fin that's broken into two sections with the lower portion being longer. (It looks like the darker fish in front might be a male.) Female anal fins are broad and unbroken. My suspicion is that the more brightly colored one is ready to spawn. Some fish intensify their colors when ready to spawn in a "Hey, look at me!" kind of way for other fish of that species that happen to be nearby. It's spring and spring gets critters into a spawning type of mood. My best guess would be the more brightly colored one was advertising their willingness to spawn. I can't see the anal fin on that fish but the one in front appears to have two different lengths to the anal fin which matches the TFH depiction of a male.