Try defining your color as a SolidColorBrush:
<SolidColorBrush x:Key="ReadOnlyTextBrush" Color="#555555" />
…
<TextBox Foreground="{StaticResource ReadOnlyTextColor}" />
Try defining your color as a SolidColorBrush:
<SolidColorBrush x:Key="ReadOnlyTextBrush" Color="#555555" />
…
<TextBox Foreground="{StaticResource ReadOnlyTextColor}" />