My Betta fish, named Sapphire, is a female veiltail and she's dark blue normally, but her color keeps fading and returning to normal in a short span of time and I don't know what to do. I already lost one to ich. Could it have to do with the water temperature? Should I change the water in her tank?
Most common reason for a betta changing colour is stress, but indeed can also be caused by illness.
If your fish changes colour regularly, and is otherwise healthy and acting normal, I think stress will be the reason.
I've had a female betta in the past, and I would notice the same every time I did my tank maintenance. She would get more pale, and a dark horizontal stripe would appear.
If my male betta was chasing her for some time, same thing happened.
But of course the 'stress' can also be caused by a bad living environment. So it is important to try to find out what the cause is.
It could also be a bad environment: Tank being too small, temperature, other inhabitants, water chemistry (pH, nitrite, ...), is the tank cycled, etc.?
Some more information can be found here Why Do Bettas Change Color?