Usually, seven-segment displays emit red coloured light, which is understandable. But in digital watches, the common colour used for the numbers displayed is black.
But black light itself can't be emitted. So the display must be absorbing light in some way instead, but I'm not able to find any resource describing the exact working of these displays.
One interesting thing I have noticed is that when I view the watch at an angle, I can view all the outlines of the seven-segment display (though it has a lot more than seven segments). But when I checked another watch that was out of charge, it didn't default all segments to black nor did it have any outlines at any angle.
I believe this means that there are some mechanisms actively working to produce this black colour. I can rule out the possibility that the inactive segments are made to give out light in the colour of their background.
I also don't think the answer is as simple as the entire background glowing extra bright, because that would be inefficient, and it doesn't change its intensity so much that we can notice it, yet the segments are very black when displaying a number.
