The same code that you are using for your Selection works for me. For example:
<RichTextBox x:Name="yourRichTextBox" TextChanged="yourRichTextBox_TextChanged_1">
<FlowDocument>
<Paragraph>
<Run Text="fdsfdfsda"/>
</Paragraph>
<Paragraph>
<Run/>
</Paragraph>
</FlowDocument>
</RichTextBox>
Code Behind:
private void yourRichTextBox_TextChanged_1(object sender, TextChangedEventArgs e)
{
yourRichTextBox.Selection.ApplyPropertyValue(RichTextBox.ForegroundProperty, Brushes.Red);
}
Once you start typing, the second letter and onward (the first triggers this change) will be red.