The paint has several colors, but there are no sharp lines or complex shapes that would have come later with the art decor bikes. Any person experienced with air brushing should be able to do this paint job, or perhaps one significantly nicer. It is difficult to capture the quality of paint feathering in a photo.

Unfortunately I don't get to see a lot of Colnago bikes up close

What is Colnago's history of the use of decals vs stencils.

My old Colnago Super (late 60's or early 70's) seems to have some characteristics that makes me think it had all of the logos painted on.
By the mid 70's, it appears as if Colnago was using quite a few decals, I think. What was the quality of the originals?
By the 90's and art decor models, I think they were back to either painting, or very high quality decal work.

So, I might go a bit by whether it appears as if the logos were just stuck on top, or are incorporated in the paint.

No doubt, a company like Cyclart does some excellent quality work, which may be difficult to distinguish from an original.

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Have you asked the seller about the history of the bicycle?

If the bike is local, at some point you'll have to go inspect the bike in person and judge it for yourself.

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Ahhh, Euros, not dollars.

You haven't mentioned where you're located.

600€ = $645.

Here in the USA, I think it would definitely be worth $645, although nice Colnago/Campagnolo bikes are still somewhat of a rarity. It is hard to say in Europe where I think they're easier to acquire.

The biggest problem is that everything indicates that the Colnago Super finished production in the early 80's, but your paint job appears to be from the late 80's or early 90's. 8-speed Campagnolo Brifters were also introduced in 1992. And, such an incongruity leaves some questions.

Nonetheless, it is a nice, and perhaps very unique bike. So, yes, that sounds like a good price, but you'll have to consider what YOU WANT as well as your local market.