Bettas have interesting scales in that there can be multiple color layers within the scale. So you add to the causes such as stress or excitement or health, the fish's genetics which can make the fish change color over time. So if you want to keep a Betta that definitely won't change color, you need to get an old fish. That said, getting an old fish means you will not have them long since they have a rather short lifespan. If you get a young fish, you will only really know what you are getting, maybe, if you can see the parents. The good news about the young fish is that you should have them for a long time and their final coloring will be a long awaited surprise.