It only has to update the parts where the words changed and it only uses the energy to initially change the screen. It literally uses more energy doing full screen wipes (flashing between black and white three or four times) to avoid ghost images. It would significantly reduce the need for a refresh if it was mostly black.
Our eyes are more sensitive to variations in lighter colors after all.
Explain eink screens
It takes more energy, and therefore battery, to draw everything but the letters and words.
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It only has to update the parts where the words changed and it only uses the energy to initially change the screen. It literally uses more energy doing full screen wipes (flashing between black and white three or four times) to avoid ghost images. It would significantly reduce the need for a refresh if it was mostly black.
Our eyes are more sensitive to variations in lighter colors after all.