What are the colour models? Is the sky blue, light blue, or maybe turquoise blue?
The human eye perceives colour differently: to one person the colour of an eggplant will seem purple, to another – burgundy, to a third – brown. But digital devices operate with precise values.
Colour models are math codes of colours. They are needed to work with colour does not arise conflicts. For example, in brand guides often indicate the brand colours by coordinates for digital media and for printing, so that they will be exactly the same on all media.
There are many colour models: they include different principles of working with colours and different opportunities to display them. You can use a different colour model, which is the most commonly used in graphic software such as Adobe Photoshop. It does not mean the colours will be same, but that the rules are the same.
Four most common colour models
LAB colour model
LAB is an oldest colour model of computer graphics. One of the earliest models that underlies Photoshop’s colour management system. The LAB model is a three-axis coordinate system:
L – Lightness, the brightness of the object;
A – the axis along which the gradations from red to green are plotted;
B – the axis with gradations from yellow to blue.
Let’s take the colour turquoise, for example, and look at it in the diagram. The L scale shows how light it is. On scale a, that it is closer to green than to red. On scale b, that it is more blue than yellow.
LAB works as the retinal neurons in the human eye encode colours. We perceive each colour based on three coordinates.
Is it light or dark?
Is it closer to green or red?
Is it more yellow or more blue?
These are called opposing signals.
The model takes as one the minimum colour difference perceivable by the human eye. That’s why Lab has a maximum colour range.
LAB is the right choice for colour correction, retouching, and print preparation. The advantage is the ability to change brightness without changing the colour values: to do this, you change the values on the L-axis.
HSB color model
When describing colour in everyday life, most of us operate with three characteristics: the colour hue, the degree of its pallor or saturation, and the degree of its brightness. This is the basis of the HSB system: there are three coordinates: Hue (colour tone), Saturation (saturation) and Brightness.
Visually, the HSB colour model can be represented as a cylinder. Saturation and Brightness range from 0 to 100%, and Hue is measured in degrees from 0 to 360.
This model is intuitive and easy to navigate. HSB is the default colour model in Adobe Photoshop settings.
RGB colour model
RGB is a model for displaying colour in digital devices such as monitors and digital cameras. It encodes each colour with the values of the base colors Red, Green and Blue. These three axes, which have a gradation of values from 0 to 255. The zero point of this coordinate system – black, and the maximum values on all three axes encode white colour.
For example, let’s see how bright red is made in the RGB system. This requires a high value on the R scale and low values on the other two.
The RGB model can be visualized as a coloured cube:
RGB is commonly used by graphic designers. This model is used by default when preparing images for digital media, because it works the same way as a monitor. The hues we see on the screen consist of three base colors, the maximum brightness of the screen is white, and the lack of brightness is black.
Even when creating layouts for label printing, the computer works mainly in RGB, and only at the stage of prepress is it converted into another model – CMYK.
CMYK Color Model
This model is based on the mixing of four typographic inks: Cyan, Magenta, Yellow, and Key (the “key” colour – black). The range of colours in printing is much narrower than on modern computer monitors. The CMYK model allows you to see on electronic devices how colors change on paper.
In CMYK model each color is coded by four coordinates, which values can be from 0 to 100%. Different shades are obtained because of the different proportions of cyan, pink, yellow and black in their composition. The white color in the CMYK model is the absence of ink.
This is what any printed picture looks like when zoomed in:
According to the ideal model, pink, blue, and yellow add up to black. Why, then, is a fourth black ink required? There are several reasons:
Dyes created with chemicals are not perfect. In practice, mixing three inks usually gives a dirty brown colour.
Colored paints are more expensive. For example, if we want a dark red colour, we can make it from red, blue and green, or we can make it from red and black. The second option would be cheaper to print.
Paper has limited absorbency. To get the closest colour to black using cyan, pink, and yellow, you will put 300% of the ink on the sheet – newsprint will not stand up to it. And pure black is only 100 percent ink.
You don’t usually edit images in CMYK. They convert the finished file from RGB or another model into this model just before printing, to check the colour match and the maximum permissible amount of colors for the desired paper type.
LAB, HSB, RGB and CMYK models are the main ones that designers, illustrators and photographers face. They simplify the work, but the actual colour reproduction is not so predictable. For example, RGB and CMYK, which are the most commonly worked with, are hardware-dependent models.
The point is that on smartphones of different generations, the same picture will differ in color, on glossy and newsprint shades will also not match. That’s why RGB and CMYK models include colour profiles that describe color as accurately as possible for a particular monitor or printing method.
Knowing the mechanics of color models is important for anyone working with computer graphics: web designers, 2D and 3D artists, game designers, and even photographers. But especially careful work with colours is needed in print design: after all, the image on the monitor to understand what the picture will turn out printed in colour on paper.
If you transfer a picture from RGB to CMYK in Photoshop, the colours would probably fade – on paper you can represent fewer colours than on the screen. But the difficulties don’t end there. Even if the layout is printed in the right color model, the result can be unpredictable, because the model does not determine the printing method and paper type. For accurate colour reproduction, you have to consider both colour models and colour profiles.