Changing Hyperlink foreground without losing hover color

Setting the Foreground directly (as you've done) doesn't work, and setting it in a Style doesn't work either, unless you "derive" that style from the default Hyperlink style (which must include the OnMouseOver trigger). So this works:

<TextBlock>    <Hyperlink>        <Hyperlink.Style>            <Style TargetType="Hyperlink"                   BasedOn="{StaticResource {x:Type Hyperlink}}">                <Setter Property="Foreground" Value="#0C0"/>            </Style>        </Hyperlink.Style>        Mark as done    </Hyperlink></TextBlock>

Extracting that style back into the Window resources and referencing it with a key would probably make for more-readable XAML, but the above code does the job.

You should build your Hyperlink like this

<TextBlock Width="Auto" HorizontalAlignment="Center">    <Hyperlink Click="ForgotPassword_Clicked">        <TextBlock Text="Forgot Password?"/>    </Hyperlink></TextBlock>

And then this style should work for you

<Style TargetType="{x:Type Hyperlink}">        <Setter Property="FocusVisualStyle" Value="{x:Null}"/>        <Setter Property="Foreground" Value="Blue" />        <Setter Property="TextBlock.TextDecorations" Value="{x:Null}" />        <Style.Triggers>            <Trigger Property="IsMouseOver" Value="True">                <Setter Property="Foreground" Value="Red" />                <Setter Property="TextBlock.TextDecorations" Value="Underline" />            </Trigger>        </Style.Triggers>    </Style>