How to change color of border in a usercontrol using trigger with its custom property?

Answer 1

I'm not quite sure what you are asking. :) But, if I could guess what you are after ... you could use a simple property trigger in a style like this:

<Style x:Key="userControlStyle" TargetType="{x:Type local:UserControl1}">
    <Style.Triggers>
        <Trigger Property="WarningLevel" Value="AllClear">
            <Setter Property="BorderBrush" Value="DarkGreen"/>
        </Trigger>
        <Trigger Property="WarningLevel" Value="Warning">
            <Setter Property="BorderBrush" Value="Yellow"/>
        </Trigger>
        <Trigger Property="WarningLevel" Value="Danger">
            <Setter Property="BorderBrush" Value="DarkRed"/>
        </Trigger>
    </Style.Triggers>
</Style>

The above assumes, of course, a enum dependency property, WarningLevel on UserControl1.