Hello Alessio,
Painting European hinges is possible but not highly recommended. First, it would be necessary to degrease the hinge to prevent the paint from pealing. Once degreased however, most manufacturers will void the warranty on the product. At Salice, we have painted hinges for customers in the past (Usually black for the office furniture industry). To keep the warranty valid, the hinge parts would be painted prior to assembly of the hinge. Once painted and assembled a very small amount of biodegradable lubricant would be applied (as always) to the hinge spring and the lifetime warranty would therefore continue to be valid.
I see by your photos that the hinges are in cabinets that probably will not be used very often by your customer. In this case, the warranty issue may not pose a problem.
Ok, now on to a better solution.
A few years ago, Salice introduced a soft close hinge designed expressly for this condition. The hinge, called Lapis, is a unique hinge system that features decorative covers of many colors including white. Not only does Lapis camouflage the unsightly nickel hinge but also adds a decorators touch with an aesthetically unique and pleasing appearance.
To substitute Lapis on this job it will be necessary to take the doors back to the shop, plug and re-paint the current hinge (8mm) dowel holes then insert the Lapis hinge in the existing 35mm cup hole. Also, check the depth of the 35mm hole. Lapis requires a 13.5mm cup depth to insure the soft close feature works correctly. The Lapis hinge requires an inline mounting plate so you will also need to also fill and paint the current mounting plate screw holes on the cabinet side.
The result will be an engineered product that your customer will appreciate.
Here is a product bulletin for lapis and a video showing the installation.
Best of luck,
Dennis