You can change the color of background images like this:-
Turns the green system image resource into dark gray:-
lay.SetBackground( "/Sys/Img/GreenBack.png" );
lay.AdjustColor( 0,-100,-60,0 );
Turns it into purple:-
lay.SetBackground( "/Sys/Img/GreenBack.png" );
lay.AdjustColor( 180,0,-45,0 );