If there is a total lack of light then we will see black or nothing. We cannot print onto a surface with light. However, not only are the CMY pigments incapable of matching all colours, their imperfections allow the reproduction of far less colour than our RGB devices. This raises many problems for the graphic reproducer. The image on the left shows a similar overlap of inks to the spotlights described above, and where an ink spot overlaps another spot we would hope to see R ed, G reen or B lue.
Process inks have to be translucent yet retain the ability to achieve set standards of density.
Where ALL three spots overlap we would hypothetically produce black but the inability of man to produce perfect pigments results in a muddy or dark brown. This fourth colour is indeed Black - often referred to as the KEY colour, hence the CMY K printing process. The image on the left comparing colour gamuts represents firstly the number of colours of light that the average human eye can detect. The second or medium sized area represents comparatively the number of colours that can be reproduced by man with RGB devices. The third or smallest area represents the colours that can be reproduced using CMY pigments.