The color subcarrier is a broadcast that is added to a black and white television signal. The color subcarrier frequency -- 3.58 MHz/NTSC, or 4.43 MHz/PAL -- is too high for a black & white TV to detect, so it ensures compatibility with such devices. Color televisions 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.