This standard for PC is early to mid 90's when ATX became common. Before ATX, there was the AT standard which had the big DIN connectors for keyboard and a DB9 serial port for mouse (sometimes the serial port was a separate ISA card). Dell started coloring the mini DIN connectors in the mid 90s
Discussion
It was actually Compaq that popularized this color coding standard, even though it was specified by IBM in 1987. Dell and gateway followed suit shortly thereafter.