$.namespace("customCss", {
    inherited: true,
    SetColor: function(color){
      return this.each(function(){
        $(this).css("backgroundColor", color);
      });
    }
  });

Without inherited param, you have to manipulate params as a array, like the example below:

  $.namespace("customCss", {
    SetColor: function(elem, params){
      return elem.each(function(){
        $(this).css("backgroundColor", params[0]);
      });
    }
  });

The use for both:

$(function(){
  $('#CustomDiv').customCss.SetColor("green");
}

Will be the next step of jQuery.NameSpace.

Thanks for the suggestion.

Marc