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https://www.lanieri.com/blog/en/a-se...n-your-pocket/
The pocket handkerchief is a chromatic accent, a distinctive sign, an accessory quirk.
It must stand out from the rest of the outfit, especially the tie: choose the right pocket square and of the same color or pattern of your tie.
The same rule also applies to pocket square and shirt, with the sole exception of the classic combination of white pocket square and white shirt.