Think of bias as the Brightness (AKA black level) control.
Think of Gain at the Contrast (AKA picture) control,
Bias or brightness is made up of a combination of RGB.
Gain or contrast is made up of a combination of RGB.
Bias affects the near black part of the image, Gain affects the near white part of the image.
Bias and gain interact.
President, Lion Audio Video Consultants Inc.
Lead THX Video Standards Instructor