CMYK Color Simulation online: RGB to print converter

Colour and print glossary

CMYK and process inks

Four printing inks: cyan, magenta, yellow and black (K for key, the plate that carries the detail). Colours are built by laying semi transparent films over white stock, so more ink means darker (subtractive mixing). Splitting an image into separate plates was already practised in nineteenth century chromolithography, and four colour printing became common in the 1920s.

RGB and sRGB

Screen colours are made by adding light: red, green and blue (additive mixing). sRGB was agreed in 1996 as the common denominator of monitors and the web. It is narrower than human vision, yet in saturated blues and greens wider than print, which is why some screen colours have no ink equivalent.

Gamut, the range of colours

The set of colours a device can reproduce. The offset gamut is smaller than a monitor in blues, greens and violets, and slightly larger in some yellows and oranges. A colour outside the gamut has to be replaced by the nearest reachable one, which is exactly what this tool shows.

CIELAB

A colour space defined by the International Commission on Illumination in 1976: L* is lightness from 0 to 100, a* runs from green to magenta, b* from blue to yellow. It was designed so that numerical distance matches perceived difference, which makes it the common ground for comparing colours across devices.

Delta E, the colour difference

A measure of the distance between two colours. The 1976 formula was a plain distance in CIELAB and exaggerated differences in blues. Today CIEDE2000 accounts for how we see: about 1 is the threshold of detection in direct comparison, 2 to 4 is visible, above 10 we speak of a different colour.

ICC profile

A file describing the colours a device or printing condition actually delivers. The format was agreed in 1993 by the International Color Consortium, so the same artwork can travel predictably from monitor to proofer to press. A profile holds conversion tables measured from a printed test chart.

Rendering intent

The rule for handling colours outside the gamut. Relative colorimetric replaces them with the nearest reachable colour and leaves the rest untouched; this tool works in that spirit, but with its own model rather than a profile transform. Perceptual compresses the whole image to keep relations between colours, which often suits photographs with strongly saturated content.

Colour separation

Converting a colour into amounts of the four inks and splitting the image into plates. The same colour can usually be built in many ways, so separation is a choice: how much black, how much chromatic ink, what total coverage. The answer depends on the printing condition and the standard in use.

Total ink coverage

The sum of all four ink percentages in the darkest area. Coated stock usually allows 300 to 330 per cent, newsprint as little as 240. Going over the limit risks smearing, set off onto the next sheet and slow drying.

Rich black

Black reinforced with chromatic inks, for example C60 M40 Y40 K100. Black ink alone reaches about L*16, a dark graphite; only a supporting layer of the other inks gets close to L*8. Small text still uses plain black, because rich black demands perfect registration.

Grey component replacement

Replacing part of the three chromatic inks with black wherever they would make a grey. It lowers total coverage and ink consumption and stabilises neutrals on press, but set too strong it flattens deep shadows. The variant limited to the darkest areas is called under colour removal.

Tone value increase

A halftone dot prints larger than it is on the plate, because ink spreads and light scatters inside the stock. A 40 per cent tint in the file typically measures around 55 per cent on coated paper. Standards define the permitted curves, and this tool computes colours with the effect already included.

Halftone screen and ruling

Continuous tone is rendered as a grid of dots of varying size. Screen ruling is the number of lines per inch: 150 for typical coated work, 175 to 200 for art books, 85 for newspapers. Finer rulings hold more detail but are harder to keep stable on press.

Moiré

A regular pattern that appears when two grids overlap at similar angles. Plates are therefore rotated against each other, classically by 15 or 30 degrees, with yellow, the least visible ink, placed in the most exposed position. Moiré also appears when scanning material that is already printed.

Spot colour

An ink mixed to a recipe and printed from its own plate instead of being built from the four process inks. It reaches colours unavailable in CMYK (vivid oranges, greens, deep blues), keeps brand colours repeatable and allows special effects such as metallic and fluorescent inks.

Overprint

A setting where the upper object prints on top of what lies beneath instead of knocking a hole in it. Black text is normally overprinted so that no white gap reveals the slightest misregistration. Overprint left on a light element by mistake can change it completely.

Contract proof

A control print made on a device calibrated to the printing condition, carrying a control strip that can be measured. It is used to approve colour before the run and remains the only reliable reference: a monitor shows colour with light, paper reflects the light around it.

Coated and uncoated paper

A coating of pigment and binder seals the pores so ink stays on the surface: colours are more saturated and dot gain is lower. Uncoated stock absorbs ink, so the print looks lighter and matt and the colour range is clearly smaller. Standards describe these cases as separate printing conditions.

Metamerism

Two samples match under one light and differ under another, because their spectral make up is different. This is why colour is judged under standardised lighting, and why a brand colour printed as a spot ink and its four colour equivalent can drift apart in a shop window.

D50 viewing conditions

The standard for judging print assumes light of 5000 kelvin and a neutral grey surround. Print measurements are reported for D50, while monitors are usually described against the cooler D65. This tool converts between the two so that the comparison is honest.

Monitor calibration and profiling

Calibration puts the display into a known state (luminance, white point, gamma); profiling records the result as an ICC file. Without it colour preview is a lottery: the same file can look entirely different on two screens. For print evaluation a luminance of about 120 candela per square metre is customary.

Soft proof

A print simulation on screen. A full soft proof needs the printing condition profile and a calibrated display (see ISO 14861); this tool gives an approximation computed from measurement data. By default it shows the colour shift alone, while a separate option adds paper white and the darkest reachable black, which makes the image look subdued. That second version matches the impression of a print lying next to a glowing screen.