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Basic Photographic Colour Theory

As far as colour in photography is concerned, there are 2 factors to consider when dealing with colour in images:

1. The Theory of Photographic Colour (the science).
2. The Aesthetics of Colour (the artistry).

Photographic Colour Theory
Photographic Colour is different to that of paints and pigments. In art class, we're all taught that Yellow + Blue = Green. That's blown out the window when it comes to photographic printing and processing, because Yellow + Cyan = Green.

The basic range (in an 8-bit space) for photographic colour is 256 colours, where 0 is Black and 255 is White. (More on the Black and White values later.)

In Photographic Colour Theory, there are 3 main colours around which it all revolves - Red, Green and Blue - known as Additive colours. They each have a pure colour value of 255:
Red = R: 255 G: 0 B: 0
Green = R: 0 G: 255 B: 0
Blue = R: 0 G: 0 B: 255

When you "mix" equal parts (value of 255) of any 2 of the 3 Additive colours together, you get 3 other colours - Yellow, Magenta and Cyan - known as Subtractive colours:
Yellow = R: 255 G: 255 B: 0
Magenta = R: 255 G: 0 B: 255
Cyan = R: 0 G: 255  B: 255

Red, Green, Blue, Yellow, Magenta and Cyan are the purest colours possible on this scale, because they are all made with 255 values.

Greyscale
Black and White points are defined as follows:
Black = R: 0 G: 0 B: 0 - therefore Black is 0
White = R: 255 G: 255 B: 255 - therefore White is 255

Other than values of 0 or 255, equal amounts of Red + Green + Blue = Grey, and this is how the greyscale is determined. Any equal amounts of Red, Green and Blue will give you a grey tone variation.

The greyscale plays two important parts:
a) It controls the density (lightness or darkness of the image), and
b) It adjusts the contrast of the image.
(As an aside: this is how colour filtration is used to determine the contrast in B&W printing – usually through the Yellow and Magenta filters, but that’s another story that I'll cover in another post at some point, because it's still applicable to digital processing!)

Here's a diagram of some photographic colour principles:



The colour star shows the Additive Colours (Red, Green, Blue) on the 3 points of the same triangle. Intersecting these colours is another triangle with the Subtractive Yellow, Magenta and Cyan on the corners. The colours opposite to each other (Yellow-Cyan, Magenta-Green, Cyan-Red) are complementary colours. In photographic terms, when Blue is added then Yellow is removed and vice versa. When Green is added, Magenta is removed and vice versa. When Red is added, Cyan is removed and vice versa A colour is made up of equal parts (255) of the 2 colours beside it. E.g. Red is made of equal parts of Yellow and Magenta; Green is made of equal parts of Cyan and Yellow; Yellow is made of equal parts of Green and Red.

Controlling colour in Photoshop comes down to 3 areas:

1.
Tweaking the additive and subtractive colours.
2.
Adjusting the colours with filtration, and/or altering the colour values with tools such as Hue/Saturation/Lightness or Colour Balance.
3.
Managing the contrast and density (lightness or darkness) of the image.

The Additive and Subtractive colours are controlled by the individual colour channels found in the Levels and Curves Adjustment tools. Clicking on the individual Red, Green and Blue channels in both these tools and adjusting the sliders (in Levels) or the curve (in Curves) will alter the individual colours, depending on how you use the tools.

The Red channel controls Red/Cyan, the Green alters Green/Magenta and the Blue changes Blue/Yellow. In both Curves and Levels, the Master RGB channel (which is actually a Grayscale where all Red, Green & Blue values are equal) manages the overall contrast and density of the image.

In Levels, if you slide the 2 end markers towards each other, you’ll increase the overall contrast of the image. Sliding the markers to the left will increase the density of the image, sliding to the left will lighten it.

In Curves, if you move the curve up above the diagonal, you’ll lighten the image. Move it below and you’ll darken the image. Move the Black and White sliders towards the center on the y axis, and the contrast of the image will increase. Move the sliders towards the center on the x axis and the contrast will decrease.

So far, all the above deals with 6 colours – Red, Green, Blue, Yellow, Magenta and Cyan (ok - 8 - if you include Black and White!) because they are all made out of values of 0 and 255.

All other colours (and variations) are made of differing values of Red, Green and Blue (on a scale between 0 and 255).

These are controlled by other tools in the Photoshop toolbox – such as the Hue/Saturation/Lightness tool, the Colour Balance tool, or Selective Colour Tool. Enhancing tools, such as the Colour Filters, Channel Mixer and B&W (when individual colours are adjusted and it’s used as a Luminosity Layer) also play a part, here.


Colour Aesthetics
Essentially, this deals with how to make an image more visually pleasing by using colours and tones which compliment each other, rather than clash. 

There’s a lot more on the web nowadays about this topic than there is about the basic photographic colour theory!

You can follow a whole host of different avenues here, for example:
1. Goethe – gave numerical values to colours (in the rainbow spectrum) so that a combination of 2 colours gave proportional prevalence to whichever colour had the highest value.
2. The Munsell System – gave each colour a value for hue, luminance and saturation, so with triangulation of the 3 values, you could find any given colour in his system. (This system is now owned by X-Rite.)
3. The CIE System – perhaps the one with the most relevance to photography because it uses values of RGB to establish the value for each colour. You do have to be aware, however, that a lot of these systems were developed for artists and pigment paints. Not all of these colours are compatible for digital output; in fact a lot of these tones and hues are not available in the colour-spaces that are available for photographic printing.

Certainly, sRGB has one of the narrowest colour gamuts, whereas Adobe RGB is larger with ProPhoto RGB being the widest out of the three.

This is another factor that you have to build into your image processing – what colourspace are you operating in, and what is your intended output - i.e. web use or print? It’s pointless getting a wonderful shade of yellow only to find that it’s out of gamut, and won’t print as it shows on your screen because it’s not within the confines of your chosen colourspace, or beyond the capabilities of your printer/inks/paper combination to accurately produce.

That’s why the CIE system would be the better one to follow – but it is more scientific rather than arty!

Here’s a link to an excellent website, and I’ll give you this page to start with:  Colour Theory (as applied to Colour Aesthetics)

To summarise:
It greatly helps to identify:1.  Which tools help to control the 8 purest colours (R, G, B, Y, M, C, Black and White), and
2.  Which tools help to control the additional tones and colour biases you can add to an image to balance it out, or to make it more visually pleasing to the eye, by balancing colour or enhancing particular colours.

Understanding how colour works in digital image processing is essential to producing great results. It's partly scientific and partly artistic. In the end, it's mostly emotional, because the response that your images receive will gauge their impact!