[old_tv_nut]

Those differences in color difference signal levels with a rainbow generator are because the R-Y and B-Y signals in a normal video signal are scaled down before modulation on the color subcarrier to prevent overmodulation on particular colors. With a color signal of real color bars or program video, those differences in gain restore the final R, G, and B signals to the right level once the luminance signal is added. For example, full amplitude saturated green bar has luminance of 59% of white, so the G-Y only has to make up 41% to turn green on completely. (The final values of R, G, and B for white may not be equal due to differences in R, G, and B phosphor efficiency, plus NTSC sets typically had increased R-Y gain to make up for the yellowish modern green phosphor.) Since the rainbow pattern consists of constant amplitude subcarrier, it does not have prescaling depending on hue, and therefore the gain differences in the color difference demodulators show up.