The blue button is there to help you adjust the phase and color controls on an NTSC television monitor. PAL and SECAM don't need it or not nearly as much.
The way NTSC (Composite) is made, the color channels from the camera are coded and squashed so they will fit on the back of a black and white television signal. If your TV has no idea what to do with those signals, you get black and white. If you have a color TV, the signals are decoded into the original red, blue and green. This is Compatible Color Television from days gone by. We're going color and you don't have to throw out your old black and white TV.
The problem with the way RCA did it, there is no reference between the original colors in the studio and the colors delivered to your house. Keep saying to yourself, "It was cheap to do it that way."
Colorbars are created with exactly the same amount of blue in the gray, magenta, cyan, and blue bars. If you press the blue button and all four of those bars are not the same brightness, then you need to adjust either the phase control or the chroma/color control. Maybe both.
You can get exactly the same effect by looking through a Kodak Wratten #98 blue separation acetate filter, which I'm sure is no longer made. It was bright blue.
A similar viewing filter was included in the last two or three versions of the Home Televsion Setup DVDs.
None of this affects the gray scale calibration. That's another whole problem. If you put one of these on your monitor, does it look gray at noon?
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