If we're trying to achieve a very dark grey tone, we must put down a large amount of all the inks - Cyan, Magenta and Yellow. This can be problematic as eventually we will be putting so much ink down that paper will no long be able to absorb that much ink - this is when the black ink comes into play. We can substitute black for a significant amount of the Cyan,Magenta and Yellow ink, and in fact create the same tone as we would have got by just mixing the Cyan, Magenta and Yellow inks together. This is known as 'Grey Component Replacement'. However, it can be difficult getting the crossover point to be smooth - that is, it can be hard to decide exactly where, and in what quantities, to use GCR as very often the visual results of this process are not perfect and the eye will see the sudden change between tones mixed from just CMY and those mixed with CMY and K.