The white lamp starts out blue

A common white LED is a blue diode of gallium nitride under a phosphor that turns part of the blue into yellow. The eye reads the mix as white.

Red and green diodes existed for decades and stayed dim or expensive as sources of illumination. Blue was the missing third, and it was a materials problem: gallium nitride, a crystal that did not want to be grown well, with the right impurities, in the right layers. Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura solved it, and a bright blue diode followed. Put a phosphor in front of it — a material that swallows a blue photon and emits a broader yellow — and the addition is white enough for a kitchen. That is the bulb.

Some lamps mix separate red, green, and blue chips instead, and the colour can then be tuned. The phosphor route is the cheap one, and it has a spectrum with a blue spike and a yellow hump, which is why some people find it harsh and why better phosphors are a real industry. The physics claim is the simpler one. The white is not a colour the diode makes. It is a colour the eye averages.