Can you show how you have assigned the usergroups color?
Usually, it takes the value from group style settings.
Enter the acp panel. then settings and groups. and edit the group of your choice. And in the "Username style" field you paste this:
<span style="color: red;"><strong>{username}</strong></span>
That is correct way to add usergroup style, however I think your plugin adds usergroup images on profile. Try to add usergroup and see if it is working out else try this plugin by Omar
https://community.mybb.com/mods.php?action=view&pid=402
It would be better if you can share your plugin link so that your issue can be understood better.