When did color-coding NM-B cable become standard in the US?
Type NM-B cable first began to be manufactured with color-coded jackets in 2001 to aid in identification of the conductor size. The color code that was introduced, which continues to be used today is as follows:

14 AWG – White
12 AWG – Yellow
10 AWG – Orange
8 AWG – Black
6 AWG – Black​

This color coding system was developed to aid those who sell, install, and inspect Type NM-B cable so that the cable size can easily be identified, to reduce mistakes resulting from the use of an incorrect conductor size.