how to change color in triggers in wpf?

I couldn't see your other codes, but you can try the following code, which works on my side.

        <Rectangle Name="rctback" HorizontalAlignment="Left" Height="100" Stroke="Black" VerticalAlignment="Top" Width="100" Margin="178,92,0,0" >
            <Rectangle.Style>
                <Style TargetType="Rectangle">
                    <Setter Property="Fill" Value="Green"></Setter>
                    <Style.Triggers>
                        <Trigger Property="IsMouseOver" Value="True">
                            <Setter Property="Fill" Value="Red" />
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </Rectangle.Style>
        </Rectangle>