how to give user feedback when he is typing something that is not in completion list

[Kristof Vansant <de_lupus pandora be>]

I'm using gtkentrycompletion in combination with gtkComboxEntry.
A list contains all user names on the system.

Now what I want is that when a user types something and it is not in the
list. The user gets an error or the gtkComboxEntry gets a different
color. Like you have in firefox when you type part of a word and no
match is found. 

So if lupus and lupin are the only items in the list. 
and the user types lupa then after pressing a the user gets an error or
something. So that he sees that he is typing something impossible.

What is the best way to do this? I'm using gtk 2.4 at the moment. But if
it is really necessary I can use 2.5/2.6

I would be very thankful if someone could help me.
 
-- 
Kristof Vansant <de_lupus pandora be>