Prof. Alexander Sahoo, University of the Arts, Bremen
Computer software usually provides too limited possibilities for colour selection. Visual colour selectors (i.e. colour patches and colour bars with sliders that represent sections through colour spaces, see Fig. 1) may work reasonably well, but they are too imprecise when it comes to the fine adjustment of individual colour nuances that are close to each other.
For small precise steps, it is best to work numerically on the computer. Here, the standard offers of most programmes are limited to a selection of the following systems: RGB (red, green, blue), CMYK (cyan, magenta, yellow, key = black), LAB (lightness, a, b) and HSB (hue, saturation, brightness). Alternatively, there are the familiar palettes for preselected colour tones (Pantone, HKS, etc.).
RGB and CMYK
The RGB and CMYK systems are device-specific systems. RGB refers to mixing from coloured light (e.g. on monitors) and CMYK refers to mixing by four-colour printing (e.g. offset printing). In relation to these technologies, they have their clear necessity. For a perceptual colour choice, however, they are rather impractical. They do not correspond to any easily imaginable cognitive model of a colour space. Or who could say, without much thought, which RGB values result in a "neutral" yellow? Since the RGB system is an additive system of light mixing, it is particularly difficult to imagine here, as our thinking is very much influenced by subtractive pigment mixing. But mixing inks also has its pitfalls: from which CMYK proportions is purple (= light violet) mixed?
Example of a colour picker (Adobe Photoshop)
CIELAB
The LAB system (CIELAB) is a device-independent colour space. One of the advantages of the LAB colour space is its four poles (instead of three primary colours). It corresponds much more to our perception to speak of a yellow tending to green or to red than of a yellow tending to blue. We can also imagine a blue-green (turquoise) better than a blue-yellow (according to Ewald Hering there are the four psychological primary colours yellow, green, blue and red). As a colour space, this system is very useful, but the parameters or coordinates a and b are very obscure. The poles of the x-axis are red and green, the poles of the y-axis are blue and yellow. A corresponds to the x-axis value and B to the y-axis value. On the Z-axis, the brightness (L) changes.
Much more descriptive is the idea of the variegated (or saturated) colours in the form of a colour circle. The hue is thus given in the form of an angle. Here, 0° corresponds to red, 90° to yellow, 180° to green and 270° to blue. For the less colourful, i.e. greyer colours, the parameter of "chromaticity" is significant. The greyer, the less colourful. The third parameter for defining a colour is brightness.
Each variegated colour has exactly one brightness of its own, yellow is quite bright and violet is quite dark. However, the larger the achromatic, i.e. grey, part of a colour, the more the hue can vary in brightness, as grey can range between black and white. A hue with a chromaticity of zero is completely grey. Here the possible difference in brightness is the greatest (from white to black).
Hue, lightness and chroma (HLC, HSB)
A definition of colour via the three colour characteristics hue, chroma and lightness corresponds to the generally accepted state of the art as used in the leading colour systems in use today, RAL Design, NCS and Munsell. For this comprehensible and easily imaginable cognitive model for describing a colour impression, the LAB colour space is measured differently or polar coordinates are used instead of Cartesian coordinates.
The colour model described here with its parameters Hue (colour tone), Croma (chroma) and Lightness (brightness) has unfortunately not been abbreviated uniformly either LCh, LCH or HLC. I prefer the order HLC (hue, lightness, saturation), because I think it makes sense to start with the hue.
The same coordinate once in Cartesian (a, b) and once in polar (H, C) notation
In many computer programmes there is a colour description similar to the HLC definition just described with the parameters HSB (Hue, Saturation, Brightness). However, this is only another form of description of the RGB colour model. Just as the LAB colour space can also be described as HLC coordinates, the RGB colour model can also be mapped with the HSB parameters. In contrast to the LAB colour space, in this model the perceived brightness changes while the brightness value of different hues remains the same (in LAB the perceived brightness of different hues with the same L-value is approximately the same; see Fig. 3). Also, the HSB model does not correspond in its hue distribution to the above-mentioned "four primary colours" according to Ewald Hering, but has the three original colours red (0°) green (120°) and blue (240°).
Comparison between the colour circles of the RGB and CIELAB colour model (HSB and HLC). The CIELAB/HLC colour wheel is more attractive because CIELAB, in contrast to RGB, is designed to be perceptually appropriate.
Missing input option
Unfortunately, the option to enter colour as HLC coordinates has hardly existed so far, mostly only as a plug-in such as CSColors! or easy.Filter ColorConverter 1.2. A direct implementation in "quasi-standard programmes" such as Adobe Creative Suite would be very desirable.
However, a major and essential shortcoming of all providers of HLC colour selectors known to me is the lack of a logically necessary limitation of the parameter values of L (lightness / brightness) depending on C (chroma / chromaticity). So far, no HLC colour selector interface is restricted to the limits of the LAB colour space. (Maximum chroma = no lightness variation, minimum chroma = maximum lightness variation). It is urgent to develop a suitable graphic interface that visually illustrates this fact.
Alexander Sahoo 2014