Changing color of a Gtk::Entry

[Johannes Bauer <dfnsonfsduifb gmx de>]

Hello list,

I've searched about how to change the color of an Gtk::Entry, but 
haven't found anything - I want it to appear greyed out, so it is 
immediately visible that nothing can be entered (it's disabled) or 
appear it with red background so the user knows he has entered invalid 
values. How can this be done?

Thanks in advance,
Johannes