Today I learned how to access StaticResource resources from xaml.cs codebehind files. This is how I'd return the regular PhoneForegroundBrush color from an IValueConverter:
return App.Current.Resources["PhoneForegroundBrush"] as SolidColorBrush;
And this is how I would emphasize text with the phone accent color.
return App.Current.Resources["PhoneForegroundBrush"] as SolidColorBrush;
Sources: source 1, source 2