Histogram

 this is a high-key image before i made the color level adjustments as you can see on the histogram.

this is the same image but with the color adjustments. it is a lot whiter as you can see on the histogram.

this is a low-key image as you can see on the histogram, it is not that dark.

 same image but i adjusted the darkness of the image. its much darker as the previous one, see for yourself on the histogram.

 this is a mid-key image. as you can see on the histogram the colors are dispersed.

this is the same image but as you can see on the histogram, all the colors are mostly in the middle. that why the call it a mid-key image.