Let's get right to the point. If you can see a second square inside the square on the left of the image above, chances are that your monitor is at least not too dark. Though, we haven't talked about colour yet (next time maybe).
If you CAN'T see a second square your monitor might be a bit too dark.
And why should I care?
Let's take the game that belongs to the screen I posted last time, it's dark and that is of course part of the idea. But during playtests some of our friendly testers mentioned that the tiles are to dark to be seen confortably and I was about to tell them to crank up the damn brightness.
They won't do it of course. I wouldn't. But then I'm on a colour calibrated monitor (ok, two of them). So for me all my colours are correct.
I guess I'm going to up the brightness of the tiles a bit then.