Link here to download a colour wheel (PDF) document
We usually think of colour as a quality inherent in an object - for example a red car or a green frog. But colour is really what we see as a result of three factors interacting: light, the object, and the observer. As rays of light hit the object, the object absorbs some light and reflects some light. We see the reflected light and perceive it as colour. Different colours reflect light of different wavelengths. Human eyes are able to perceive thousands of colours in the visible spectrum of light.
When you apply ink to paper, the colours we see result from the light that the ink reflects. Computer monitors use emitted light rather than reflected light. The colours we see result from light emitted from the screen.
To describe how colour is produced or perceived, we use colour models. Computer monitors display colours by producing varying amounts of red, green, and blue light - the RGB colour model. Human eyes perceive colour by its hue, saturation, and lightness levels - the HSL colour model.
RGB is the default colour model in GIMP. GIMP uses your preferred colour model (RGB or HSL) whenever colour values are displayed. When you select colours from the Colour Picker, you are able to enter RGB or HSL values regardless of your preference setting. The colour model used to display colour values on-screen has no effect on how colours are printed.
All colours on your computer screen are created by mixing red, green, and blue light in varying proportions and intensities. When these primary colours are mixed in equal proportions, they create yellow, cyan, and magenta. Adding all the colours together creates white.
Each primary colour (red, green, and blue) is assigned a value from 0 (none of the colour present) to 255 (the colour at full strength). For example, pure red is produced by combining a red value of 255, a green value of 0, and a blue value of 0. Yellow is a combination of a red value of 255, a green value of 255, and a blue value of 0. Setting all three values to 255 produces white; setting all three values to 0 produces black. When all three colours are set to the same value, the result is grey.
The HSL model is based on how the human eye perceives colour using the characteristics of hue, saturation, and lightness. Each characteristic is assigned a value from 0 to 255.
The three characteristics are described as follows:
Hue - the colour reflected from an object, such as red, yellow, or orange. Each hue value is assigned based on its position on the colour wheel. On the Colour Picker’s Colour
wheel, colours are assigned anticlockwise from the top. Red is at the top (value 0) and as you move around the wheel the colours go through orange, yellow, green, blue, purple, and back to red.
Saturation - the purity or vividness of the colour. Saturation represents the amount of grey in the colour, from 0 (entirely grey) to 255 (fully saturated colour).
Lightness - the perceived amount or intensity of light in the colour. Lightness ranges from 0 (no light, or black) to 255 (total lightness, or white). At 50 percent lightness, or a value of 128, a colour is considered pure. For example, pure red has a hue of 255, a saturation of 255 (100 percent) and a lightness of 128 (50 percent). For pure blue, the hue is 170, saturation is 255 and lightness is 128.
CMYK Model (The GIMP Doesn’t By Default Support CMYK Read More Here).
The CMYK model is based on the fact that ink on paper both absorbs and reflects light. As white light strikes the ink, part of the colour spectrum is absorbed and part is reflected back to your eyes (resulting in the colour you see). (Download GIMP’s CMYK Here).
In this model, the primary colours cyan (C), magenta (M), and yellow (Y) combine in varying proportions to produce a variety of colours. When the three colours are combined, they produce black. Because impurities in the ink make it difficult to produce a true black, a fourth colour, black (K), is added.
Combining inks in this way is called four-colour process printing. It is used by printing services and high-end colour printers.
CMYK channels are four separate greyscale images that represent the percentage and location of cyan, magenta, yellow, and black in the image.
When you print CMYK separations, (Photoshop, for example) prints a separate greyscale page for each primary colour. You can then use these pages as colour plates when working with a printing service.
Wendi E. M. Scarth. Top of Page.