Colour video signal does not extend beyond about 1.5 MHz. 

Therefore, the colour information can be transmitted with a restricted bandwidth much less than 5 MHz. 

This feature allows the narrow band chrominance (colour) signal to be multiplexed with the wideband luminance (brightness) signal in the standard 7 MHz television channel. 

This is achieved by modulating the colour signal with a carrier frequency which lies within the normal channel bandwidth.

 This is called colour sub-carrier frequency and is located towards the upper edge of the video frequencies to avoid interference with the monochrome signal.

Bandwidth of colour signals is restricted to about ± 1.2 MHz around the subcarrier.

Below figure gives necessary details of the location of monochrome (picture), colour and sound signal spectrum all within the same channel bandwidth of 7 MHz.

It may be noted that in the American television system where the channel bandwidth is 6 MHz, the colour sub-carrier is located 3.58 MHz away from the picture carrier.