[Koroku]

In the Usergroup Manager, there is a section
Username HTML Markup
(Specify an opening and closing HTML tag with which to display the names of users in this group. This applies to names that appear on Logged-in Users, Who's Online, Users Browsing Forum, Users Browsing Thread, Posts, Member List etc.)

In the first box, put
<span style="color: red;"> (replace red with whatever color)

And in the second, put
</span>

And it should work good