In Transmission line, Why the Insulator color is always brown?

In Transmission line, Why the Insulator color is always brown?
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I'll do some research on the internet but can't find anything exactly about the color of the HV insulators.

Consulting one Chemical Engineer I get logical answer ... Porcelain with low quality has yellow pumpkin color ( which is the case with first porcelain insulators ).
Manufactures colored with brown color because of that ...