jQuery Css Selector

jQuery Css Selector

I have a question about jQuery and Css Selectors...

My HTML is:

  1. <div id="rev_slider_3_1">
  2.         <ul style="Something">
  3.            <li style="Something">
  4.                <div class="slotholder">
  5.                    <div class="tp-bgimg defaultimg" style="background-color: for example blue"></div>
  6.                </div>
  7.            </li>
  8.            <li style="Something">
  9.                <div class="slotholder">
  10.                    <div  class="tp-bgimg defaultimg" style="background-color: for example red"></div>
  11.                </div>
  12.            </li>
  13.            ...(A few more li's)
  14.          </ul>
  15.     </div>
I want to change the div's (tp-bgimg or defaultimg) background color using jQuerys .css method...
I also want to have the background colors alternating ....

So I first tried to change the bg-color for one div...

by using this code:
  1.  jQuery(".tp-bgimg").css("background-color","red");
But it does not have any effect.... :(

The URL is:  http://hoellen.de/wp/

the div I want to change is the div with sliders content in it (bg-image is slider image)...

I hope someone could help me..

Best regards