Usually this happens because a background color has been set in the css and then either an image or a different background color in the css overrides it.  When the page loads the yellow css is rendered first and then overridden.  I get this a log when I am developing because I will drop a red background color into highlight what I am working on till I get the CSS down and then remove the red background.  Sometimes I forget to remove the red though and it shows up on page load.  I would look for a yellow color code in your css somewhere.