SOLUTION FOUND

instead of using the dynamic resource directly in tint color i used a 'zombie element'

<Label x:Name="Ref" BackgroundColor="{DynamicResource MyColor}" IsEnabled="False" IsVisible="False" />

and then used it as a refrence in TintColor

<toolkit:IconTintColorBehavior TintColor="{Binding Source={x:Reference Ref}, Path=BackgroundColor}" />