Paint concrete prefabricated garage white / light gray / gray?

That may be so. Two concrete surfaces, plaster, and possibly other surfaces are offered to choose from and priced individually. That is not a big deal and more transparent than some all-inclusive offers with only one total number. I find that completely fine; in the end, an individual selection (size, material, roof, drainage, surfaces, interior fittings, gate, drive, side doors and windows, etc.) is needed, which is then compared as a total price. The interesting question now is whether the concrete can be chosen without an extra charge and painted by oneself to save the cost of the plaster without damaging the exterior of the garage, e.g., through rain, etc.

What does the garage manufacturer say about his standard surface?

Is it then finished or does it still require on-site post-treatment?

How is the surface quality of the concrete surface? It should be at least fairly good if you only want to paint.

Concrete surfaces Interior: surface hand-trowelled; exterior: surface formwork-smooth. The surfaces correspond to exposed concrete classes SB1 and SB2. Alternatively, coating with roller plaster, standard white, or for on-site facing, plaster or paint possible.