Nearly half of the passenger airplanes flying today are leased by the airlines from big plane leasing companies. It is easier and cheaper to lease a standard white airplane and then just change its logos than repainting the entire plane (which some leasing companies may not even allow).

White has some significant thermal advantages over color.

A white object reflects all wavelengths of light, so the light is not converted into heat, while a coloured object absorbs more wavelengths of light and converts them into heat, so the object gets warm.

In most cases, this is probably just a benefit in terms of keeping the cabin slightly cooler. In the case of ‘plastic’ airplanes (those built with composite construction), some airframes require the use of white paint on upper surfaces to keep some elements within limits. Early Diamond Aircraft designs had a 38° C outside air temperature limit, past which the main spar is not considered structurally sound.

Also Read: How is aircraft’s landing gear made?

Concorde had to be painted with  a special, highly reflective white paint to mitigate the extreme heating effects that friction caused at mach 2.

This could be the reason how white became the standard color.

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White is a nice, neutral base color.

Artists typically start out with a white canvas, or white paper – it’s a color associated with “blank” and “clean”. Starting with a white base coat allows the airline’s marketing department to paint whatever livery they want on their flying billboards.