(Another distinguishing characteristic was that NTSC's dependency on accurate phase meant that it was difficult to maintain the color as the signal was transmitted and processed. Television engineers often joke that NTSC stands for "Never Twice the Same Color.")
The NTSC standard adds a color subcarrier which is quadrature-modulated by two color-difference signals and added to the luminance signal. The genius of the system is that black and white TVs ignore the color components, which are beyond the black and white signal's bandwidth.
The NTSC color subcarrier reference is 3.579545MHz. The horizontal sync rate (H) was adjusted slightly from the black and white standard's 15.750kHz such that the color subcarrier is 455/2 times H. The vertical rate is Fv = Fh x 2/525.
See: Video Basics.