If you go into the panel properties to the "Background" tab and click the
"Background image:" radio button while the file chooser button says "None" then
the panel will revert to showing the colour from the system theme.

A lot of applets, though, will show whatever they were showing last (ie: not be
updated at all).  This includes quicklounge, the window list, gweather and the
clock (although the menu bar correctly shows the system theme colour).

This problem is obvious to the user when switching from "Solid colour" with
transparency since the desktop pixmap is left as the background of the affected
applets.

This probably isn't libpanel-applet, but I'm not sure where else to put it.