I am so sorry to hear that he passed.
Many Bettas do change color just as they grow to adulthood. With all the new breeding and variety of colors, they have more marble genes which get such a wide variety.
As for the fish and stress, making sure the water is in top condition is a priority.
Doing regular water changes is important. Testing the water quality is also critical to keeping the water in the safe zone.
The article linked in my signature gives a bunch of helpful hints for keeping fish happy in their tanks.
FW & SW
Favorite tank right now - 72G Bowfront Wadai Tetra, Cardinal Tetras, Ghost Glass Cat Kryptopterus bicirrhis and Clown Loach Cory haven
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article