Mixing coloured light, where combining colours moves toward white — how screens work.
Additive colour mixing combines light of different wavelengths; red, green and blue light combined at full intensity produce white, which is the basis of the RGB model used by digital displays.
Additive mixing (light) and subtractive mixing (paint/pigment) behave in opposite ways — mixing all additive primaries gives white, mixing all subtractive primaries gives near-black.
A phone or TV screen produces white by combining red, green and blue light at full brightness.
Additive vs subtractive mixing is the core reason screen colour and paint colour don't always behave the way people expect when combined.