There is a tutorial on how to use colours:
http://www.msgplus.net/help_tutorials.php?tut=multi-format-colors
AND
http://www.msgplus.net/help_tutorials.php?tut=contactlist-colors
IF you want to stop colours showing:
How can I disable colored names in my contact list?
You can disable the use of colors and text style if you wish, this can be done in the Plus! preferences panel.
1. In your Messenger main window (contact list), select the "Plus!" menu, and then select "Preferences...". A new window, called "Messenger Plus! - Preferences" will appear.
2. On the left side of the "Messenger Plus! - Preferences" window, select the "Contact List" section, you will now see all the general contact list options.
3. To disable colors and text style in the contact list, uncheck the box in front of "Display formatting attributes in the contact list". Check the box to enable this feature.
4. Press "OK" when you're done.
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