In this case, if the result of the P0 expression is zero or null, P1 will be effected. Otherwise, P2 will be.
<tab name="Xamarin">
<source lang = "xml">
<Controls:EvalExpression x:Name="WFcolors_Issued" Expression="(P0==0 || P0==null) ? P1 : P2">
<Controls:EvalExpressionParameter Name="P0" Value="{Binding Values[SC_STATUS]}" />
<Controls:EvalExpressionParameter Name="P2" Value="{StaticResource MATAP_Priority_Green}" />
</Controls:EvalExpression>
</source>
</tab>
</tabs>