I couldn't find the right answer in the world wide web, so I'm thinking about this one only on my own.
Let's see. What is the difference between black-and-white prints and colour prints? You can express more with different colours, show anything you want and point at something by just using some special colour. In black-and-white you have to deal with just black colour (because we usually print on white paper). Colour prints are more useful in business and demand on them is much higher. You can use colour prints in many way and they will look much better than just black prints.
So, the demand on colour prints is much higher. I'm not quite sure, but we can call colour prints and black-and-white prints close compliments, because if the price on colour prints rises really high, the demand on black-and-white prints will rise, because lots of people would have to switch to them.