Changing prefix

Below the items section, you can see a section named "menu"

  menu:
    title: "[Bounties]"

if you want to change it just replace the text in brackets with anything you want.

By default, the colour of the prefix will be RED, if you want to change it put the reset symbol §r and then colour referring to the Minecraft text formatting table:

Text formating table

For example, if you want the prefix to be Bolded and gold, the beginning of the prefix should look like that:

§r§6§l

do not use & symbol, right now it will show a crash message in the console