Surcharge for light gray exterior facade ok?

The ban on surcharge for light tones from 1871 applies only on the left bank of the Rhine in predominantly Protestant communities.

For the sake of completeness: ...and only on days that end with G or H....

The additional charge probably comes from the fact that your facade is only plastered white as standard and not painted white.
Therefore, an additional work step (2 coats of paint necessary or colored plaster with [Egalisationsanstrich]?) for a large area.

well, the contract says white and not light gray. He will probably have to buy paint that is more expensive

The surcharge probably comes from the fact that your facade is only plastered in white as standard and not painted white.
Therefore, an additional work step (2 coats of paint necessary or colored plaster with an equalizing coat?) for a large area.

That must be it!

Fairness only exists in sports.
You can try to negotiate, but you have an extremely weak negotiating position.

If it is actually about the surcharge for colored plaster vs 2-3 coats of paint, you can of course inquire elsewhere (using the existing scaffolding).

Our construction service description states the following:
"The mineral exterior plaster is therefore applied as a white premium plaster on a full-surface base coat. To prevent cracks, different types of substrates are properly reinforced. The surface is textured by being scraped out with a grain size of 2-3mm."

It will probably be similar for us, also a local general contractor. White is stated in the contract; verbally, and thus negligible, we were told that a slight darkening or another light shade might be possible – so rather no, negotiating won’t bring anything in this case.

What caught my attention in the sentence from your construction service description is: "To prevent cracks, different types of substrates are properly reinforced."
For me, that means you do not have a full-surface fabric plastering, which is also the case according to our contract, costs about €4,000 more according to our GC, and we will probably have it done.

"May" was perhaps an unfortunate choice of words. Of course he IS ALLOWED to charge that, the question is more about how fair it is and whether we might be able to negotiate that ;)

This is not about fairness. He makes you an offer and you can accept it, but you don't have to. You have a choice. You could also have the work done by someone else if you want. "Fairness" here implies that you are being ripped off. But that is not the case.

Why didn't you address this right at the beginning during the contract negotiation?