Blue is part of how screens make white
Smartphone screens are not generally designed to maximize blue light. They are designed to produce bright, convincing colors while using little power—and blue is one of the three basic colors they need to do that. A screen combines red, green, and blue light in different proportions to create the colors you see, including white.
On many LCD phones, a blue LED helps generate the screen’s white backlight. A layer of material converts some of that blue light into other colors, and filters in each pixel shape the light into red, green, and blue. OLED screens work differently: their pixels produce colored light directly. In both designs, blue is essential to the display’s color output.
The engineering tradeoff
Blue light has a relatively short wavelength, and producing it efficiently is a key part of making a screen bright without draining the battery quickly. Display makers also need enough blue output to create crisp-looking whites and a wide range of colors. But that does not mean every screen pumps out unusually high levels of blue: brightness, color settings, display technology, and what is on screen all affect the light reaching your eyes.
Night or “warm” display modes reduce the screen’s blue component by shifting whites toward yellow or orange. That changes the color balance rather than switching blue off. The tradeoff is visible: warmer settings can make a screen more comfortable to look at in a dark room, but they also make colors less accurate.**
