function choose_colour(inner_or_outer){
  var chooser = document.getElementById('court_select_colour');
  chooser.data=inner_or_outer;
  chooser.style.display='inline';
}
function click_colour(colour_code){
  var chooser = document.getElementById('court_select_colour');
  chooser.style.display='none';
  var inner_or_outer=chooser.data;
  var image='url(/tennis_court/images/tennis_court_'+inner_or_outer+'_'+colour_code+'.png)';
  var layer = document.getElementById('court_'+inner_or_outer);
  layer.style.backgroundImage=image;
}