Key Colour Patterns
It has been a long-held view that the CMYK colour space is better to work with than RGB as there were established rules to follow.
FIGURE 1 Lists of CMYK values are sometimes used giving the colour levels at different densities for neutral tones, white skin tones, dark skin tones, etc.
As there were no equivalent lists for the RGB colour space it therefore apparently followed that RGB is more awkward to work with; that results are more of a hit and miss affair.
But rules do exist for the RGB, and ones that are easier to learn at that. In fact using RGB allows much of the mystic of colour correction to be removed so that most people are able to quickly check on the colour health of images, even if they are not directly involved in doing the colour correction themselves. Much of these basic rules are based on the photographic knowledge that many colour values are pivoted around the colour green, which covers a range of colours from greys, sky blues, to skin tones.
FIGURE 2 One of the advantages of using the RGB colour space is that neutral grey tones are of equal value in the Red, Green and Blue channels.
With black the RGB values are all 0, white RGB values are all 255, and any equal values in between are the greys from dark to light. This means that the colour sliders form a verticle "|" shape pattern making it very easy to see if any grey has a colour cast.
FIGURE 3 Blue sky is useful even though it is a gradient from dark blue at higher levels to light blue nearer the horizon, the colour pattern is uniform.
Strictly speaking the sky colour is Blue Cyan as the pattern is one where the Blue value is high, the Red value low, and with the Green value in between the two. In the darker part of the sky the pattern is more to the left of the Color Palette, and further to the right as the sky colour is nearer the horizon. The sliders form a "\" pattern with an angle of approximately 45°, but this is not always so and in the lighter part of the sky the angle is steeper as the blue becomes desaturated.
NOTE: Deliberate exaggeration of blue sky is a common problem in images. While this can sometimes make an image more dramatic, it often results in an unrealistic sky colour that can be very difficult to print.
FIGURE 4 Skin tones are extremely useful for gauging image colour. Humans are very sensitive to skin colours, even if we are not normally aware of it.
We are not too worried about changes to sky blues, so long as they do not become green or have a purple cast. However, if skin becomes too Blue, Red or Grey then things start to look very un-natural to us. This has probably been acquired during evolution as a natural warning that a person with these skin colours is probably cold, ill or even dead. Anyway, the skin tone pattern is one where the Red value is high, the Blue value low, and with the Green value in between the two. This is true regardless of the basic racial colour, with white skin values more to the right of the Color Palette, and black skin values more to the left.
Basically, skin tone patterns are the reverse of blue sky. Whereas sky blues have a "\" pattern, skin tones have a "/" shape instead. Deciding which parts of the skin to read the colour values from is more difficult than reading sky blues. Typical problems are cheeks and noses being too pink, under chins reflecting local colour, and cosmetics used on the face. Good places to read values from are between the eyes, on the temples, or the arms and legs, assuming these areas are not too red from sunburn.
FIGURE 5 Here we see how the colour of leaves has a greater amount of variation within one scene than does grass.
Grass tones can sometimes be used as a way to measure the image Green values. However, they are more difficult to use as grass can vary greatly depending on the seasons. Fresh grass can be bright green with extra yellow. Later the green becomes darker, then during a dry season will likely be more yellow than green. Nevertheless, as a general rule the average grass colour pattern is one where the Green value is high, the Blue value low, and with the Red value in between the two.
This colour pattern can sometimes also be applied to trees, but as can be seen in the image above, the colour of leaves has a greater amount of variation within one scene. Although the pattern for green grass is not as useful as those for grey, sky and skin tones, it can still sometimes be of help and so is worth remembering. While the grey tones, sky blues and skin tones have straight line patterns of "|", "\" and "/", the grass green is different in that it has a non linear ">" shape instead, but with the Blue value lower than the Red.
FIGURE 6 Some surprising exceptions to the rules.
Like most rules, there are the exceptions. Greys cease to be neutral when the light source ceases to be white. The classic example of this is at sunset where so much of the blue end of the spectrum has been scattered by the atmosphere that the sun light becomes very red.
Blue sky changes with altitude with the range between dark and light becoming exaggerated. Views from high-flying aircraft can reach a point where the sky directly above is black, even with stars being visible, but still be blue lower down through to white at the horizon. This is because the distance to the horizon is far greater as altitude is increased, and so the blue becomes more and more desaturated.
Skin tones can quickly become difficult to judge when cosmetics are used, when skin is sun burnt, or infected.