BackgroundColor is not applied property when DynamicResource is used and button enabling state is changed
<Style TargetType="telerik:RadButton" x:Key="MyTelerikButtonStyle">
            <Setter Property="TextColor" Value="{DynamicResource MyPrimaryColor}" />
            <Setter Property="BackgroundColor" Value="{DynamicResource MySecondaryColor}" />
            <Style.Triggers>
                <Trigger TargetType="telerik:RadButton" Property="IsEnabled" Value="false" >
                    <Setter Property="BackgroundColor" Value="Gray" />
                </Trigger>
            </Style.Triggers>
</Style>WORKAROUND:
Use StaticResource:
 <Setter Property="TextColor" Value="White" />
 <Setter Property="BackgroundColor" Value="Green" />