Let's see if this helps..
Professor- professes
Contractor- contracts
Juror- does whatever a juror does.
Calculator- calculates

Colour- does nothing
Honour- does nothing
Flavour- does nothing
Armour- does nothing

The u aids distinction between solid, identifiable nouns, the ones you can point at, and the ones that don't have a presence of any sort.

Armour being an exception, because this is English, and all rules need an exception.
( Thinker, Fri 2 May 2008, 18:47, Reply)