I think an argument can be made for the difference between the stop timer, and the stop stopwatch. Red (in those two cases) means stop, so the button turns red

I think an argument can be made for the difference between the stop timer, and the stop stopwatch. Red (in those two cases) means stop, so the button turns red. Stopping a stopwatch is an expected action to perform, so the button remains white (to be consistent with start). Stopping a timer is an unexpected action—you are cancelling the timer. To denote this, they use dark grey.