WPF中让TextBlock每一个字符显示不同的颜色

<TextBlock x:Name="tb">
              <Run Foreground="Red">R</Run>
              <Run Foreground="Green">G</Run>
              <Run Foreground="Blue">B</Run>
              <Run Text="Gradient">
                 <Run.Foreground>
                    <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                       <GradientStop Color="#FF000000" Offset="0"/>
                       <GradientStop Color="#FFFFFFFF" Offset="1"/>
                   </LinearGradientBrush>
              </Run.Foreground>
         </Run>
</TextBlock>

string str1 = this.tb.Text;
string str2 = "";
foreach (Run r in tb.Inlines)
{
    str2 += r.Text;
}
MessageBox.Show("str1:["+str1+"]         str2:["+str2+"]");