1st of all black color is a dark color that can absorb all kind of color. tyre always will be in touch of the ground and ground is not clean everywhere sothe black color always absorb the color of the other item wrapped around the tyre. so it dont look separately otherwise your car and bikes wud look so bad. think of a car with white tyre and mud is all around the tyre. black would be your choice then...
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If u need scientific reason that is, the raw material by which tyre are made.
Imagine all the tyres are in White... Is it looks good?