Why do the Gold and Silver Brothers in Naruto Shippuden have whisker marks? I recently watched episode 269, and in a flashback, they had whisker marks BEFORE they got swallowed by Kurama (Nine-Tails) and started eating its flesh.
My guess is the author is making a reference to the Maneki-Neko or the fortune cat with the whiskers. It is a tradition that these ornaments brings forth good luck or fortune. Back in the old days, copper, silvers and gold were used as part of the currency exchange, that is why the Maneki Neko is sometimes coated in gold like color. 

Towards the end before the Gold and Silver brothers were completely sealed away, Darui did mention something about if a bunch of copper work together, it is worth more than a lump of gold or silver.