How to get Background colour for a Widget

[Hywel Thomas <hywel controlspecials demon co uk>]

I'm trying to get the current background colour for a widget, with odd 
results.  I'm using the Gtk# bindings, so I'm not sure what the calls 
translate into in Gtk+, but if I read a DrawingArea (da) Background with:-

Gdk.Color bg = da.Style.Backgrounds[(int)StateType.Normal];

then set the colour back to this value with

da.ModifyBg(StateType.Normal, bg);

I get a different colour from the original!  If I set the default BG 
with the same call, but with a null colour argument, the original 
background is indeed restored, but I seem to be unable to read this 
colour.  The obvious property for the BG colour is 
da.Style.Background(StateType.Normal), but this property is write-only 
(though I'm not sure why).  Any help in understanding what is going on 
would be most welcome.  I've tried the Gtk# list, but it was suggested 
I'd do better here.

-- Hywel Thomas