I think for GA black is better for matching the instrument bezels, and white legend text stands out nicely. You can compare this with this. I was going to screen print the text in white (or whatever colour) but it wasn’t very visible on the grey – same as the original Socata labels.
Airliners and pressurised planes generally use grey (with white text) which is probably easier on the eye for many hours, but they all have much smaller windows and less light getting into the cockpit.
I tend to disagree on this one, because all our gliders have flat grey instrument panels, and those have as much light and window area as one can possibly get.
The tone seems to be even less glossy than the panels on the pictures.
An interesting snippet of information from another part of the globe: Soviet aircraft, both civilian and military, used to have black or sometimes gray panels, but from mid-1960s they switched to teal, baby blue, or steel blue, allegedly for ergonomic reasons.