[old_tv_nut]

Another factor is that color-under systems used automatic video gain control on the luma and chroma to try to maximize the signal to noise ratio on low contrast and/or low chroma scenes, without overloading on high contrast and/or high chroma scenes. The noise level at the output could vary depending on what AGC characteristics were in the recording machine, and how much of the scene had high chroma levels.

EDIT: The playback circuit had to detect the video level coming off the tape and compensate it back to normal. The automatic RECORDING gain control was taken advantage of by CopyGuard, which put video pulses in the vertical blanking that varied from zero (black) to super-white, thus causing the record circuits to turn the video gain down and making the picture fade out periodically when you tried to make a copy. (The machine playing back the original CopyGuard protected tape ignored the vertical pulses.)