Color Theory Basics - Key Color Imaging Terms

In practice, color images and portions of color spaces are often described by three terms: Color professional typically use, (1) neutral gray (neutrality), (2) tone reproduction (contrast), and (3) color correction (hues of an image).

The first term neutrals refers to neutral gray image areas. The second term tone reproduction identifies the contrast of an image. The third term color correction identifies the hues of an image such as red, green, blue, cyan, magenta, yellow. Each of these three image areas are comprised of highlight, midtone and shadow areas. In color separation, CMYK halftone values create each of these image areas.

Neutral Gray

Neutrality refers to a range of neutral gray image areas that are seen as gray tones. Gray tones within images vary from light to dark and do not appear like colorful hues of red, green, blue, cyan, magenta, or yellow. Neutral highlights appear white, shadows appear black and midtones appear in various shades of gray. Photoshop has specific features that control neutral areas. An alternative term for neutral gray is gray balance.

Measuring Neutral Gray

Neutral gray is used as a reference point for reproducing color images and is a good place to start measuring an image. It is the easiest image area to define. Neutral highlights appear white, shad-ows appear black, and midtones appear in various shades of gray.

Three different counting scales are used to measure images:

• Density Values
• Digital Values
• Halftone Dot Values

Measuring neutral gray in originals, such as transparency density , and digital images on a color monitor is a straight forward concept: Equal amounts of RGB density or digital values achieves neutral gray.

• Density values range from 0.0 (the lightest area) to 4.00
  (the darkest area).
• Digital values range from 255 (White) the brightest area, to 0 (Black) the darkest area.

Creating Neutral Gray with CMYK

Inks Using CMYK inks to create neutral gray with halftone dot values is not a simple concept. Equal amounts of cyan, magenta, and yellow does not create neutral gray. The results of that combination achieves a pinkish gray. To achieve neutral gray with cyan, magenta and yellow a special relationship of the halftone dots must occur.

Determining the correct gray balance for an image reproduction (CMYK) is visually compared to the neutral image areas of the color original and a color proof or printing. Gray balance is determined by trial and error and is usually unique to the scanning and image reproduction system.

This chart illustrates typical neutral gray values for RGB and full range CMY images. The gray balance values shown above and below are typical for printing and provide a good starting point to establish gray. As each color system is fine tuned, these values are slightly altered to compensate for the intricacies of a color reproduction system.

The following chart references neutral gray values for RGB and CMY. The values are measured at the Photoshop default Preferences settings.

Tone Reproduction

Tone reproduction is a term used to describe the contrast of an image. This is one of the most important image processing adjustments. If the image’s contrast is correct, then other color correction problems don’t seem as difficult to deal with. The tone reproduction charac-teristics are identified by image areas such as highlights shadows and midtones. Other tech-nical terms are interchangeable to describe image contrast. Other terms include but are not limited to, gamma, tone shape, gradation and luminosity.

Terms Related to Tone Reproduction:

• Highlight - the whitest area of the original and the reproduction.
• Shadow - the darkest area of the original and the reproduction.

When highlight and shadows are set correctly the lightest and darkest image areas will visually make the reproduction look like the original. This will make the final reproduction highlights and shadows will have the correct amount of white and black details. If highlight and shadow areas are not adjusted correctly each reproduction will be inaccurate visually.

Midtones

• The midtone areas are between the highlights and shadows. Adjusting midtones achieves the correct contrast in the reproduction. Midtones affect the amount of lightness and darkness of both the neutral and color areas.
• One of the main factors for achieving a good color reproduction is to identify the distribution of tones within an image. Since many variations exist between each type of image category, a simplified approach to reproducing different types of images is to establish three basic classes:
  1. Average images are not too light or too dark. It contains an even distribution of all levels of tones and colors. are those with equal amounts of light and dark tones.
  2. Light images or High key images are light looking images. A light image means the color original consists mostly light tones and colors. predominantly have light tones.
  3. Dark images or Low key image are dark looking images. A dark image means the color original consists of mostly dark tones and colors.
  

A popular term often used to describe image class is "keyness". The context for keyness is high key for light images and low key for dark images.

Adjusting Light and Dark Looking Color Images

There are varieties of light and dark image categories and each image has its own unique distribution of tones. This means only experience will provide exactly how much to adjust a midtone, but the principles of adjusting midtones for average, light and dark images are the basis for achieving the optimum contrast in a color reproduction.

To understand how to adjust and work with light and dark images, it is necessary to first to establish how an average image is reproduced. (i.e. establish the average image parameters). The average image parameters establish a starting point for reproducing all types of color images. The average setup is based on reproducing most average original images with little modification. To use this strategy highlights and shadows are defined and adjusted with little change and are applied with standard midtones settings, pre-established neutral values and hue and saturation (color correction) values.

Once color parameters for average images are identified, the approach is to use them as a starting point to adjust light and dark images.

Guidelines for Adjusting Highlights, Shadows and Midtones

Neutrality is a good guide for determining where and how to adjust highlight, shadow, and midtone areas from the start of the image reproduction process.

To begin work with an image, adjust highlight areas first.
Find a neutral white area that carries the whitest image details.
This is called the diffuse highlight.
Use target values of cyan = 5%, magenta = 3% and yellow = 3%.

Work with shadow areas second.
Find the darkest neutral black of the image area.
If using the Photoshop default settings use target values of:
Cyan = 65%, Magenta = 53%, Yellow = 51% .
The Black value is adjustable usually between 80% to 100%.

Midtone adjustments are based on two factors:

1. Image content which measures if the image is average, light or dark.
   
   High key images require increasing midtone areas to bring out highlight to midtone details.
   
   Low key images require decreasing midtone areas to bring out the shadow details.
   
2. Neutral gray image areas.

Color Correction

Color areas of a reproduction make up the hues of images, such as red, green, blue, cyan, magenta, and yellow (RGBCMY) Each color area has tones of hues that range from light to dark. Though generally not characterized by highlights, shadows, and midtones, color areas are directly affected by highlight, shadow and midtone adjustments. The terms most commonly used to identify color image areas are color correction or selective color correction. Photoshop has specific features that control selective color areas.

What Values Make Up Colors?

Guidelines for Determining Colors when Scanning or using Photoshop Below are examples of how colors would typically measure in the Photoshop Info Palette. These are just guidelines, because various images have different degrees of hue and saturation.

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