Color changes with aging?

[RandomWiktor]

So, I've noticed in pretty much all of my bettas that they seem to undergo some color changes - often times profound - as they age. I'm wondering if this is because most of mine were sick when I got them but are now healthy, or if this is overall pretty common.
Examples:
- Lug was a dull, brick red color on his body and fins when I got him. He is still that color, but now has blue streaks in his fins and is developing iridescent green scales on his sides.
- Tiny Tim has periwinkle bars on his gills (previously all red), has developed dark, almost black markings on his side, and blue streaks in his red fins.
- Dickey Barret, who is blue with a pinkish/flesh colored head and upper body, has developed red (almost purple) streaks in his fins.
- Pulpul, whose color I could best describe as "peach," has developed a very reddish face as he aged, and also has random red patches on his fins and back (not disease related, they're honestly just coloration).
Many of my previous bettas also had this, such as Sealink, who started out looking a green-ish brown color and ended up iridescent purple

Anyone else, or do I just have magical color changing fish?

 

[Dart]

I've had some changing colour bettas, but not quite to the extent you've had

I had a little red guy develop purple rays in his tail. And my blue betta has gotten a lot darker then when i first had him.

 

[FishyLoveBug]

Horizen was a very light blue when I brought him home. Now he is a darker blue with green metalic. If you are using color enhancing food that may also be the change you are seeing.

 

[ThatDarnDragon]

my no named fish was bright red with blue tips on fins, now he's solid crimson red.

my others stayed the same

I guess they do change with age?

 

[Synirr]

Almost every single marble betta I've ever bought has changed colour with age, so I'm going to assume that it's common
Raziel in my sig has much more red on his anal fin now than he did when that photo was taken.