Composite video is an "older" type of video signalling allowing
all the information to be transmitted on a single coaxial cable or
radio frequency carrier. It is a combination of amplitude
modulation with phase modulation of a common color subcarrier
frequency. The brightness is coded in the amplitude of that
subcarrier and the color is coded in the phase of that subcarrier:
red is one phase (e.g. 0 degrees), green is a phase 120 degrees
away from red (e.g. 120 degrees), and blue is a phase 120 degrees
away from green (e.g. 240 degrees). Composite video also requires a
color burst during every horizontal sync period to provide a phase
reference to prevent color drifting over time.
It is not clear from your question what combination of red,
green, and blue you are describing.
If the red, green, and blue magnitudes are identical (producing
white) then the color subcarrier turns off and the brightness of
the white is simply a "baseband" black & white signal (possibly
even just a DC voltage). But if any color is to be produced the
color subcarrier turns on with a phase somewhere from 0 degrees to
360 degrees, with each phase angle being a different color on the
spectrum from red to violet (just like seen in a rainbow).
Very few computer monitors use this system due to limited color
resolution and thus poor display quality on larger screens.
