Those polygons are the ones that are displayed when no other polygons are displayed, they are usually called "background" and , of course, they cover all the map. I don't understand why you write that it is not the background.
If no other polygons are displayed and if those background polygons are not displayed too (or are transparent) , then the screen is black.

So, to change this yellow back color, you only have to add this background polygon to the list of polygons and to the draworder with the value 1. And finally you have to increase all other draworders to 1 (only the background must have draworder=1)