I would like to be able to change the colour of the mentioned name by using a hex code. So in config an option like MentionColour where you can put a colour code. At the moment the only option is to do it by groups etc.

I don't see an option for this being added. The purpose of the mentioned color is to highlight the player's user group. If you'd like to change what color the player is mentioned as you can change their BetterChat group color in the respective config file.

I may add a config option to make all mentioned names a static color.