How to increase the contrast of a colour image?

Please find the below method. It is the most easiest way. You can set the maximum and minimum values and adjust the image to that region.

low = 0.3;
high = 0.7;
K = imadjust(I,[low high],[]); % I is double

Another way of contrast enhancing is by doing histogram stretching. you can use histeq (see doc histeq for more details), or you can stretch it manually.