Use ColorAnimationUsingKeyFrames to animate the foreground color of a TextBlock

   
      

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Grid TextBlock.FontSize="192">
        <TextBlock Name="txtblk" Foreground="Black"
                   HorizontalAlignment="Center" VerticalAlignment="Center" />
    </Grid> 
    <Page.Triggers>
        <EventTrigger RoutedEvent="Canvas.Loaded">
            <BeginStoryboard>
                <Storyboard TargetName="txtblk" Duration="0:0:2" RepeatBehavior="Forever">
                    <StringAnimationUsingKeyFrames Storyboard.TargetProperty="Text">
                        <DiscreteStringKeyFrame KeyTime="0:0:0" Value="textBoxA" />
                        <DiscreteStringKeyFrame KeyTime="0:0:1" Value="B" />
                    </StringAnimationUsingKeyFrames>

                    <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground.Color">
                        <DiscreteColorKeyFrame KeyTime="0:0:0" Value="Red" />
                        <DiscreteColorKeyFrame KeyTime="0:0:1" Value="Blue" />
                    </ColorAnimationUsingKeyFrames>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </Page.Triggers>
</Page>