Given the choice, i am a fan of the black dials stock to the base 718's. I would preferably opt for the black dials had I ordered an S model. For me, the grey dials are more difficult to read, plus the black dials look more sporty than grey. I've heard many people mention negative comments on the grey S dials all across the Porsche range. The colored dials (red, yellow, etc...) look really cool when matching exterior or interior bits, but are also not as functional to read as the black dials...
I didn't know what you guys were talking about at first until I realized I have a base car with black dials. Did not know other versions have grey or other colors for the tach. Mine's easy to read.
One thing that may help is if you adjust the "ambient lighting". This is a very subtle interior lighting that is always on when driving. You can darken it and perhaps, that will help with instrument contrast. Worth a shot.
This was a common complaint for the 'S' models previous to the 718 as well. Not sure why Porsche decided to retain the feature considering the issues ... probably because it's an option to order dials in alternate colors such as red or yellow, so if one really has an issue with it, you can pay extra to nullify it, ya know?