Cellophane betta getting dark fins?

So I have this betta. He is old, around 4 or so now. He is also a father to a 14 week old son, who is bi color purple, after his mother. Now Kai, the old man, always used to be perfectly skin toned. Over the past few months, it seems his fins have started getting darker colored, like a brownish side to them. It is not fin rot colored, though he may have a little of that that I am working on treating, and he is completely happy and healthy and relaxed with just a little mystery snail as a tank mate, getting fed daily and some occasional live blood worms as a treat (he doesn't eat the ones out of the jar. *sigh*). But why would a 4 year old betta just start changing color? I have had him for about 3 years now (give or take) and he has always been well taken care of, and never changed. Is it like when mammals get old they get gray hair? He is still pretty, albeit starting to show his age and reduce some of his energy. He is next to his son but the little one looks like a very small little female right now, about 1/3-1/2 an inch and still short fins that he is developing, and the father hasn't seen the mother since breeding back in May, so there is nothing interesting to encourage his colors to enhance such as competition or a female. He shows no interest in his son besides occasionally looking at him out of curiosity. Any ideas for this change?