In offset printing, a spot colour is any colour generated by an ink (pure or mixed) that is printed using a single run.
The widely spread offset-printing process is composed of four spot colours:Cyan, Magenta, Yellow and Key (black) commonly referred to as CMYK. More advanced processes involve the use of six spot colours (hexachromatic process), which add Orange and Green to the process (termed CMYKOG). The two additional spot colours are added to compensate for the ineffective reproduction of faint tints using CMYK colours only. However, offset technicians around the world use the term spot colour to mean any colour generated by a non-standard offset ink; such as metallic, fluorescent, spot varnish, or custom hand-mixed inks.
The dot gain, hence the screen angle and line frequency, of a spot colour vary according to its intended purpose. Spot lamination and UV coatings are sometimes referred to as 'spot colours', as they share the characteristics of requiring a separate lithographic film and print run.
Process coloursThe term "process colours" refers to CMYK colours. They're called process colours as they're the colours which are involved of every printed material, no matter what print process you use, be it an inkjet or a professional scale lithographic printing job, you will always see the printers have separate cyan, magenta and yellow inks forming in varying levels layer by layer to form an image.
Pantone
Pantone is a colour matching system used to ensure that colours remain as consistent as possible, regardless of the device/medium displaying the colour. Pantone matching system is used by the printing industry to print spot colours. Most applications that support colour printing such as; photoshop, illustrator and indesign allow you to specify colours by indicating the Pantone name or number. This assures that you get the right colour when the file is printed, even though the colour may not look right when displayed on your monitor.
Duotone
Duotone is the generic name for multi tone printing, which can be done with two, three or four inks. This process requires that the press be set up with special inks, usually PANTONE-designated colours, instead of the standard CMYK inks used for process colour printing. Usually the images are printed with a dark base colour and a lighter second colour, overprinted to fill in, tint and tone the photo or graphic.
The Pantone matching system works well for spot colours but not for process colours, which are generally specified using the CMYK colour model.
Halftone
Halftone is the ‘industry standard’ reprographic technique. It simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing. A ‘halftone’ is also used to refer specifically to the image that is produced by this process. This reproduction technique relies on creating a basic optical illusion—that these tiny halftone dots are blended into smooth tones by the human eye. Colour printing is made possible by repeating the halftone process for each subtractive colour—most commonly using what is called the ‘CMYK colour model’. The resolution of a halftone screen is measured in lines per inch (lpi). This refers to the number of lines of dots in one inch. The higher the resolution being used, the greater the detail that can be reproduced.
CMYK
The CMYK colour model (four-colour process) is a subtractive colour model. Primarily used in printing, CMYK works by partially or completely masking colours on a white background. The printed ink reduces the light that would otherwise be reflected. That’s why this model is called subtractive because inks ‘subtract’ brightness from a white background from four colours: cyan, magenta, yellow and black.
CMYK is able to produce the entire spectrum of visible colours due to the process of half-toning. In this process, each colour is assigned a saturation level and minuscule dots of each of the three colours are printed in tiny patterns. This enables the human eye to perceive a specific colour made from the combination. In order to improve print quality and reduce moiré patterns, the screen for each colour is set at a different angle.
Tints
A tint is a shade of colour that has been diluted through the addition of white making it a more transparent version of it's original colour. Tints are only available through the spot colour process because they use the same plate and ink as the original. The way tints are achieved is through applying less of the ink in increments of 10%, therefore, it's not actually 10 different colours but the same colour, just 10 different tints. The use of tints allows us as designers as it opens up more possibilities without affecting or increasing costs.