I have taken a video clip of what I see here. 

You will have to look closely as you will see that there is a datagrid which loses it's header's color and also alternate rows background color.  Everything turns gray.

The datagrid is actually a third party control from Infragistics.  I doubt I can get anything of value from their tech support if I showed it to them.

 So what could possibly make this happen? 
 

It looks like it could be a css issue, possibly with the modalbackground css class. Try removing that from your modal popup to see if that eliminates the problem. You can also try using the IE developer toolbar to inspect the css classes of the underlying grid and maybe pinpoint the conflict.