Bacground color of tabbed button

[Sarah Reichelt]

Hi All,

If I have a tabbed button on OS X,  I get a nice 3D border around a
space of a slightly different color to the background of my stack.
This is a nice effect, but how can I find out what the color of that
area is?

I need to put an opaque field on a tab's background, but I want it's
background to have the same color. It's opaque becasue it is hiding
other things, but I want it to look as if it is transparent.

The background color for the stack can be changed by the user, which
chnges the background color of the tabbed button, but by varying
amounts, so I can't hard-wire either a color or a color difference. I
just need a way to find the real color of a certain area of the
window.

TIA<
Sarah