Chroma Subsampling

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance.

It is used in many video encoding schemes — both analog and digital — and also in JPEG encoding.

Read more about Chroma Subsampling:  Rationale, How Subsampling Works, Sampling Systems and Ratios, Out-of-gamut Colors, Terminology, History, Effectiveness, Controversy