ASK ANDY




Color Models and Matching Systems

I've recently received a question about colours, via the email link on the web site:


Hello Andy,
I am investigating the basic differences between a colour model system and a colour matching system. Also, could you please tell me of a colour model that is used in a computer package and one which is used in the printing industry.
Thanks
Paul Willcox

Thanks Paul for the question. You need to know the differences between a Colour Model system and a Colour Matching System.


A Colour Model system is a means of describing a colour by way of a mathematical formulae. The colour is stored simply by remembering values representing a particular variable in the equation. There are several popular colour models, and some are based towards 'on-screen' output, some 'printed' output, and some output independent.


RGB Colour Model:

An example is the RGB Colour Model, used predominantly in computer monitors and TV's. The amount of each of the Red, Green or Blue light required to create a given colour is stored as a number between 0 and 255, 0 being the minimum (no light) and 255 being the maximum (brightest) light.

Varying the amount of each of the Red, Green and Blue values creates individual colours, some examples: black R0 G0 B0, white R255 G255 B255, blue R0 G0 B255, mid grey R128, G128, B128, yellow R255 G255 B0, orange R255 G128 B0. Because of the nature of monitors or screens (using three R, G, B colour sources), there are all based on the RGB colour system. (Changing colours on my Win/PC uses the RGB system).


Hue, Saturation and Brightness Model:

Another system is HSB. This is based around Hue, Saturation and Brightness, and a colour is described by storing a value for each of this three variables. Hue ranges from 0 to 360 degrees and represents the position of the colour on the standard colour wheel, itself based on the colour spectrum. Saturation is the amount of hue in the colour, and the brightness is the amount of white light in the colour. Depending upon the exact system, the values for the saturation and brightness may be 0 to 255 or stored as percentages (0 to 100%).

Examples of HSB:

black=hue: doesn't matter, saturation: doesn't matter:, brightness 0% the hue and saturation are not relevant because the brightness is at 0% so there is no light or colour, giving black strong bright blue= hue:200 degrees, saturation: 100%, brightness:70% reducing the saturation to 30% gives a more washed out blue or blue grey, reducing the brightness to 30% gives a very dark blue HSB Colour model is 'device independent,' it's not based around the working of either printers, or monitors and screens, it represents the colour purely theoretically.


CMYK=Cyan, Magenta, Yellow, and Black Colour Model:

The Cyan, Magenta, Yellow, and Black Colour Model is based around printing techniques, and is also known as CMYK or 'four colour printing process'. This is the model most full colour printed materials are printed with (newspapers, magazines, books etc.). With the other models mentioned, they use amounts of light, with CMYK it's amounts of each C, M, Y, or K ink. On a monitor black is created by using 0 red, 0 green, 0 blue, but on white paper, 0 red, 0 green, 0 blue would create white. Therefore the 4th ink is used and is black, also better results are obtained with Cyan, Magenta and Yellow, rather than Red, Green and Blue ink.

A colour in CMYK is represented (and printed) by the percentage amounts of each ink, some examples:


orange= Cyan 0%, Magenta 50%, Yellow 100%, blacK 0%
red= Cyan 0%, Magenta 100%, Yellow 100%, blacK 0%
green= Cyan 80%, Magenta 0%, Yellow 80%, blacK 0%
blue= Cyan 100%, Magenta 75%, Yellow 0%, blacK 25%

The CMYK model is very device dependent, in other words, depending upon the kind of printing press used, and the type of paper (low quality newsprint for a daily paper, or high quality coated paper for a glossy magazine), these values would change. For a newspaper the paper is more pourous, and 'runs' more, so less ink would be needed, so red may be 80% magenta, 80% yellow as 100% of each may be too much and saturate the paper with too much ink. For a high quality glossy the paper could handle 100% of both magenta and yellow.

A colour model system is the way the colour is represented and remembered, a colour matching system is a system to allow different colours using different models to be represented by one another. A 'colour gamut' is the range of colours from the colour spectrum, that a particular colour model can represent, and the colour matching system tries to match different colours using different models, with different gamut, to each other.

The RGB model can create many more colours than are available in the CMYK model, so there are many RGB colours that can be created and views on screen (RGB) but cannot be printed using CMYK. Therefore the RGB model has a much greater 'colour gamut' than the CMYK model, and the matching system would warn of 'unprintable' RGB colours.

For example, if you use desktop publishing software to create a document that will be finally printed on a printing press using the CMYK four colour process, you will have created your colours with various amount of CMYK. However this needs to be displayed on screen too, to represent you document, and anything 'on screen' is represented by the RGB colour model. So the matching software is having to create the impression of a CMYK colour, using the RGB model (CMYK monitors don't exist!), and also warn you of any unprintable RGB colours.

Another area of colour matching system is that of creating accurate colour across many different printers and devices. Imagine you create a colour in a document that you are going to print on a printing press, and specify the colour (orange) as exactly Cyan 0%, Magenta 50%, Yellow 100%, Black 0%. It may be that the printing press is not so accurate and may gain or lose a few percent, so you end up with 48% Magenta and 98% Yellow. Then again, if you print 'test' examples on a desktop computer printer, it may be that the 50% magenta becomes 55% and the yellow, say 90%.

How can you rely on your 50% and 100% being exactly that when printed? This is where the colour matching system comes in. The manufacturers of printers, screens, and matching software can test how accurate their systems are. In the case of the desktop printer, you'd need a whole range of test colours, and using a densitometer, measure the actual printed density of ink. The test document could tell the printer to print say 5%, 10% 15%... etc. right up to 100% of each C, M, Y, K ink. Then the actual printed ink densities are measured and compared to the document 'true' values. This shows if the printer is 'magenta heavy' or 'cyan light' etc., and the software can be instructed to compensate for the printers variation in ink density.

The values may look like this:
Colour in document: 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Colour on printout: 8% 23% 29% 41% 55% 61% 71% 78% 88% 95%

Looking through the values, the colour matching software would compensate for the losses and gain, whenever using this printer. If the value needed was 10%, then the matching system can see that this would be printed as 8%, so it sends 12% knowing that this will more likely print at 10%. Each % value could be tested and stored, so that a given % can be compensated for. Once created, this table would be stored for that particular printer, and used whenever that printer was used. Are you still with me?

Every device (monitor, software package, printer, even paper type, etc.) used could have it's own colour compensation table. Using a different printer, the software uses the table for that printer. This method is then taken further, so that a printer can represent different kinds of printing press output, simulating various kinds of paper and so on. CMYK is very 'device dependent' and using a different 'compensation table' can allow the same desktop printer to estimate or simulate how a document would print on printing press. Using these kinds of colour matching systems can help to maintain colour accuracy across a wide range of printing devices.

As to a colour model used in a computer program, that would be the RGB model mentioned earlier, and one for the printing industry would be CMYK mentioned earlier too. There is another area to colour matching systems; books of colour samples with specific ink values and printing press specifications are available. You simply use the colour swatch book to match the kind of printing press output you're using, chose the colours from the book, and use the exact C,M,Y,K values of any colour in the book that you like. Those colours were already printed using those values, so if you find a colour you like, you simply use the same values and you'll (hopefully) get the same colour on print out (even though it'll probably look different on screen).

I hope this goes some way to explaining colour models and matching systems, it's a subject that can go on forever!

If you have any DTP or graphic design related questions please feel free to contact me on the email address below. Until next month...
Seasons Greetings, Andy.

Contact: Andrew.Davidson@onyx.octacon.co.uk




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