For many years I would get questions about the legitimacy of a resident card by clients and employers. The reason is that for many years the so-called Green Card wasn't actually green.
In May of 2010 Immigration started issuing a resident card that actually was a "Green Card".
Today, you are most likely to encounter one of the two cards below both of which are legal proof of Permanent Residency.
USCIS began issuing the most recent version of the green card in May of 2017. There are many previous versions of the green card that are still in use and they are valid until the expiration date that is on the card.
The New Version of The Green Card (Front)
The New Version of The Green Card (Back)
The Previous Version of the Green Card (Front)
The Previous Version of the Green Card (Back)
These cards often do not contain a signature. A signature is not required for the card to be acceptable