I see now how it's done, but it's done blind as when setting the Intro Screen background color they do not show the slide screen. Not too good.
OK, I now see what you mean - the image is immediately replaced by the Intro Screen colour. It should operate like Color Wash, in the Backdrop panel.
As a workaround, I'd suggest sampling from one of the other places where the image is displayed. So you can sample in the Navigator panel, the filmstrip, or even a second window.