Q. What do colors of output harness mean?

A． Red, White, Yellow,...they are very colorful.
As you know, PC has several output voltages including 5V. Design Guide for Desktop Platform Form Factors Revision 1.31 specification defines the color of Input/Output as follows,

Also, as individual company and product may use another color assignment, you need to check
individual specification (labels, etc.)
However, as to Red for +5V, Yellow for +12V, and Black for COM, they are normally in common use.