1. An automatic gain control apparatus comprising: means for providing an input image signal from each of a plurality of separate color channels; a plurality of variable gain amplifiers, each coupled to receive an input image signal from one of said color channels and provide an output; a plurality of low-level selecting means coupled to receive an input image signal from one of said color channels and the output of the variable gain amplifier receiving an input image signal from the same color channel and for providing an output image signal which is the lower level one of the received input image signal and the corresponding variable gain amplifier output signal for that color channel; high-level selecting means coupled to receive said plurality of input image signals and responsive to the said input image signals for providing an input image signal output having a level equal to the maximum one of said input image signals; and gain control means responsive to said input image signal output from said high-level selecting means for controlling the gains of said variable gain amplifiers to vary a knee slope of the amplitude transfer characteristic of each of said low-level selecting means in said separate color channels when the peak level of at least one of said input image signals exceeds a white clip level, in such a manner that the knee slope prevents the output image signals from exceeding the white clip level.