Spot colours are inks that do not belong to the CMYK colour space, but are printed as a real colour in an additional inking. The most important representatives are PANTONE, HKS and TOYO colours.
As a bright CMYK Red is produced by overprinting 100% Magenta and 100% Yellow inks, a spot colour, such as PANTONE Warm Red, is printed as a real colour in its own inking unit, and therefore can achieve a higher colour gamut than the mixed CMYK colours. Luminous colours like Pantone 811 or metallic colours like silver and gold can only be reproduced by spot colours.
The disadvantage of spot colours lies in the higher costs. A booklet with a PANTONE spot colour and colourful images has to be printed using 5 colours: CMYK plus PANTONE red. This requires 5 printing plates and a printing machine with 5 colour stations. The advantage of higher colour space is so often contrary to the disadvantage of the higher cost.
Spot colours can be reproduced very well in modern proofing systems. The colour variations of the proofs of Proof.de are published here and mostly reflect the PANTONE and HKS Colours to be well within the achievable Proof.de Gamut.