The color subcarrier is the signal added to a black and white television signal to add color information. The subcarrier frequency -- 3.58 MHz/NTSC, or 4.43 MHz/PAL -- is too high to be detected by black and white televisions; this ensures compatibility with such devices. Color television sets employ special circuitry which detects and decodes the color component for display.
The chrominance subcarrier is a separate subcarrier signal that carries the color information during the transmission of a video signal. It is modulated and synchronized using the color burst signal and then attached to the back porch of the color composite video signal. By synchronizing the subcarrier with the local oscillator of the television receiver, the RGB colors can be decoded successfully.