As you use the Mac, opening windows and dragging them around, you may have noticed that the color of the windows can shift tone and change hue, sometimes fairly dramatically so, and depending on what your MacOS wallpaper is set to. For example, if you have an orange wallpaper as your desktop background picture, the windows on the Mac will be tinted orange as well. It’s not quite a transparent effect, but more of a translucent effect where only the color of the wallpaper will bleed through to tint the windows in the foreground.
This feature is referred to as wallpaper tinting in MacOS, and you can turn off this window color tinting feature on the Mac if you’d like to.
Read more at OSXDaily.com