Hi Richard - a couple of observations:
Firstly, what a great shame that the sterling job you are doing with the car has been messed up by a fairly elementary error. As you say, surely they should have questioned the different colour straight away?!
Secondly, as previous owner of the car, I'm not aware of any change of bonnet and I don't recall there being anything in the history file that I passed onto you. I was pretty content that the front end of the car was original. And surely if it was a new bonnet with a different paint code sticker, the bonnet would have been a different colour!
In terms of colours I think AR 755/AF Grigio Nisida is the correct colour for a 1986 Green Cloverleaf Sprint. Tim's Alfasud page backs this up, though without the numerical codes:
"Buyers of the Sprint 1.5 QV could only chose between the colours Rosso Alfa, Bianco Capodimonte, Nero and Grigio Nisida metallizzato while the Sprint 1.3 was available in further metallic colours (coming from the range of colours of the Alfasud series III) in the beginning of its career.
With the launch of the Sprint 1.7 QV three colours remain in the range: Rosso Alfa, Nero and the new tone Bianco Argento metallizzato."
I think Bianco Argento is the sort of 'champagne' colour that the later 1.7 Cloverleafs came in (with tweed cloth trim) - is this what they have sprayed it with?
Below are what I think Grigio Nisida and Bianco Argento look like:
There are 10 types of people in the world: those who understand binary and those who don't.