Hi,
I need to change username color only for 20 users. I cannot do it with primary usergroups. They are in the second
You would either need to make core changes or a plugin.
Set their display group to the secondary group.
Make a new group. Name it whatever the other group is called, and place these 20 in the new group. then you can make changes for them, and people can't see it's a diffrent group.