This happens to me all the time. If I am in “Play” mode in the editor and I click on “Pause”, I am inclined to click on “Play” to continue running my game. Of course, this immediately stops Play Mode instead of un-pausing.
Since that button functions as “Stop” while in Play Mode, the icon should change to a stop icon.
I understand your issue, but I am afraid that not many people share this problem with you. I think this is just something you will need to get used to 
Happens to me as well. It would be more logical to update the play icon to a stop icon when in play mode. The Unity manual says the editor UI will darken when in play mode but the change to the UI is so slight you barely notice it. A simple change of the play icon shape would be more user friendly.
You can adjust the color change manually. In preferences → Colors you find a setting called Playmode Tint. This is used as the color overlay when in play mode.