I have just tried to add a user to the Global Moderator group, and after I did, their username color has stayed the same.
I have the color set 00AA00.
Can someone help?
You need to set it as their default group ---> Users and groups ---> Manage users ----> Enter the username ----> Select groups in the top drop down box and click Make group default for member next to the global moderator group and he will be that colour.