In order to change the foreground color of the RibbonWindow's title you can alter the template of the title through the TitleBarStyle property of the window. You can take a look how to extract a template in our Editing Control Templates help article. Basically you can set the WindowTitle style and change the Foreground property of the TextBlocks inside its ControlTemplate. Here is the style:
For your convenience I extracted the template and prepared a sample project.
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