HKS is a German color system, which was developed by the companies Horstmann-Steinberg, Kast + Ehinger and H. Schmincke and Co., hence the name: HKS. It is quite common in Germany, but plays nor role internationally and is increasingly driven by Pantone color as a global system to the brink.
HKS is available in four different color guides:
HKS K for coated paper
HKS N for uncoated paper
HKS Z for newsprint
HKS E for rotogravure
The original 88 colors were a few years ago expanded with the new HKS 3000+ color fan to 3520 colors that do not really have different colors as opposed to Pantone, but have more shades through overprinting the actual colors with black. By doing so, 39 shades per color are generated.
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.