White light isn’t real. It’s an illusion created by a combination of the visible wavelengths. A rainbow splits sunlight into its myriad colours.

While your ears can identify various instruments playing simultaneously, your eyes will interpret different combinations of colours as similarly white, even if they only contain a tiny fraction of the full spectrum.

Your brain is incredibly clever. It will adjust for the actual colour of the light in your environment. Therefore, a sheet of white paper will look white whether you’re inside with artificial lighting or outside in the sun.

The camera isn’t so smart. It objectively records what you show it. Film is usually optimised for daylight, so it will look blue if you use it in the shade (because you’ll be lit by the blue sky), or have an orange colour cast if you use it inside under incandescent tungsten lighting. Film photographers use coloured filters to change the colours.

So the colours can be cooler or warmer depending on the predominant wavelengths of light.

Holistic Photography White Balance Warm Cool Colour Scale

Digital cameras are marvellous. You can tell them what should be white or neutral in your scene, and they’ll change all the colours to match. This is the White Balance.