Use Default Name Color Behavior (/color Default)
The default behavior for name colors seems to be that it temporarily chooses a random color for you each time you join chat, but this behavior is permanently lost once you've chosen a color. There's no option AFAIK to regain this functionality - the only way to apparently have this chat "color" is to use an account that's never chosen one.
Hello there, you can change your name color in chat in your chat identity settings! More information here: https://help.twitch.tv/s/article/chat-basics?#settings