@kdgalla, again, thanks for your reply.
This is a new project, made with the Core 3D builtin render pipeline template. I haven’t imported anything like ProBuilder or (knowlingly) changed any settings. I have imported a rather small bunch of assets, inluding the checkered material visible on the plane in Game View, though, but I am pretty sure that is not the issue. Also, it is definitely not the directional light - all lighting is off-ish
I have tried to toggle skybox visibility - not working either.
However, I may have found the color. And possibly the culprit. Not the cure, as of yet.
Apparently there is a toggle box in upper edge of Game View window - named “Light Settings” - which normally is not there. I have inserted another screenshot in the OP. Only, it doesn’t toggle anything, but the colour is shown when it is activated. That togglebox did appear in all the various new projects I created to investigate, and it seems to be something akin to the wireframe rendering thing. It shows something about which surfaces receive what lighting. I am not able to remove or disable the box, and as I said there is no response whatsoever from trying to toggle anything. I have no idea where it came from, but somehow it inserted itself into the layout of the window upon starting a new project. It does not seem to be connected to the “Light Settings” submenu from the Rendering window. I don’t know what it is. But it came back in every new project I tried to create.
And the good thing is - while working on the problem yesterday, I failed to notice that the powercord had disconnected. So this morning, my laptop had run out of power, and when I reconnected and restarted, somehow the default layout settings had been restored and the togglebox was not there when I tried once more to make a new project. It is still present in the 7 different test projects from the previous days, but they are expendable. (and yes, I definitely have tried to restart both the Hub, the Editor and the computer before writing the OP).
So - problem not solved, but it can be ignored, for now. Still - explanations would be welcome. Yesterday was four very embarrassing lessons with students due to derailed preparation…