You cannot change the font color of tabs but you can change the flash color.
Messenger Plus! Live - Registry Settings
In the above page, it explains how to set registry settings. The key's name you must create and set is TabChatFlashColor. It makes the tabs flash with the color you choose. For example type FFFFFF as the value of key, and it will make it flash with white color.
Please read the paragraphs of that page carefully before asking any question about registry settings.
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