As television signal was originally a single brightness channel, there was a need to add colour in a way that is compatible with the older standards. This way it is possible to watch colour transmissions on a B&W TV, and B&W transmissions on a colour TV.
Three components are required for transmitting colour, but the brigthness signal is needed for compatibility. So RGB is translated to three other signals, one that is equal to the B&W brightness Y, and two other signals that are called the colour difference signals, U and V.
In NTSC colour transmission, the two U and V are slightly processed (scaled and rotated by 33 degrees) to end up with two similar signals called I and Q.
Basically U&V, or I&Q, is a sine (or cosine) wave at the color carrier frequency (3.579545 MHz) which encodes colour in the amplitude of the sine wave (indicating saturation, how much colour there is) and in the phase of the sine wave (indicating hue, which colour there is). The phase reference, the colourburst, is sent on each line before video.
So in short, two colour signals are quadrature amplitude modulated at the colour carrier frequency and added to the brightness signal with reference to end up with the composite signal.
Thus the single composite signal does not contain the three YIQ/YUV signals directly, but in encoded form. So composite must be decoded into the three YIQ/YUV signals before converting back to RGB for display.