Colour concepts

Colour model

Colour model or colour mode is a way to define colour: it describes how colour will appear on the computer screen or when printed.

Some colour models are:
RGB
HSB
CYMK
Greyscale

The RGB (Red, Green, Blue) colour model

This colour model is used for colours that will show on TV screens or computer monitors.

A value between 0 and 255 is assigned to each of the colours – Red, Green and Blue - and the resulting colour is made up of the combination of them.

For a purely blue colour:  Red is 0, Green is 0 and Blue is 255 (pure blue).
For Black: Red, Green and Blue are 0
For White, Red, Green and Blue are all 255 (white light is made up of all colours on the spectrum, black the absence of them all).

The HSB-Hue, Saturation, Brightness colour model

Hue 

The the "basic colour" of something, measured as an angular value on the colour wheel.

Saturation 

This measures the purity of the colour, also called  chromaticity, especially in TV and video work.
A yellow that can't get any yellower is fully saturated.
To desaturate a color add grey.  Or think of it as subtracting colour and leaving grey behind (use which-ever description is more intuitive).

Brightness 

This is determined by how much black is mixed with the colour.
Colors aren't all perceived as being the same brightness, even when they are at full saturation, so the term can be misleading. This on it as the z-axis of a colour cylinder.

CMYK-Cyan, Magenta, Yellow and Black

This colour model is used for print work. It describes colours according to percentages of Cyan, Magenta, Yellow and Black.

Commercial printers uses these four colours to reproduce full colour artwork in magazines, books and brochures. By combining them on paper in varying percentages, the illusion of lots of colours is created.

Grayscale

In Grayscale mode an image is made up from different shades of gray in an image.  Every pixel has a brightness value ranging from 0 (black) to 255 (white) - so in an 8‑bit greyscale  image, there can be up to 256 shades of gray.

In 16-and 32‑bit images, the number of shades in an image is much greater than in 8‑bit images.

Grayscale values can also be measured as percentages of black ink coverage (0% is equal to white, 100% to black).




See this video for a view on how the colour models are different in what they can represent:

Colour depth

This is the number of bits which are allocated to store the value of a colour which is displayed.  The more bits per pixel, the more colours that potentially can be displayed  Each bit has a value of 0 or 1, so one bit can represent two colours.   N bits can represent 2^N colours so:

1 bit = 2 colours
4 bit = 16 colours   (= 2x2x2x2)
8 bit  = 256 colours
24 bit = 16,777,216 colours

Printing equipment today cna usually handle up to 8 bit colour - display screens can go far higher.

Colour Balance

This characteristic of a multi-colour image has a high value when the relative density (luminance - should this be luminescence) of all colours in the image is about the same and no one colour dominates.

Contrast, Brightness and Gamma

Brightness

Traditional brightness brightens the entire image from the shadows to the highlights equally.

Gamma is an alternative, non-linear, method of adjusting brightness.  It works by changing the mid-tones of the image only, not the highlights. Increasing Gamma can make an image look brighter, but it does so by increasing the brightness of the shadows and mid-tones, not the highlights.

Contrast

Contrast is the separation between the darkest and brightest areas of a image.

Increasing contrast increases this separation, so shadows are darker and highlights are brighter.

Decreasing contrast brings the shadows up and takes the highlights down, so they become closer to one another.

Adding contrast usually adds "pop" and makes an image look more vibrant while decreasing contrast can make an image look duller.

Transparency

In Photoshop, all layers, except Background layers, are transparent, ie have transparency capabilities.

Even if a layer is filled with pixels, eg with a full size image, or a fill colour, the layer itself has a transparency value, and if this is set to a high value, or if  pixels on the top layer are erased or masked, it allows other layers below to show through.

You can give any layer a degree of transparency by adjusting its opacity:   high opacity = low transparency, ie they are the inverse of each other.

If necessary, the background layer can be converted to a regular transparent layer.