Colour modes & image management

The colour you see on a computer monitor is not the colour you will see on a printed page. Different methods of printing are used for specific requirements (and quantities) and together with varying stock types, colour will often appear quite different to that seen and selected via the computer.

The first colour basic to understand is the difference between how a monitor portrays colour and the method used to transfer a file into a printed document. On the monitor, primary colours of light (additive primaries) are generated by different degrees of electrons. The mix of red, green and blue light (RGB) is perceived by the eye as the full RGB spectrum of colour.

On paper, cyan, magenta and yellow (subtractive primaries) are mixed with black (adding crispness to dark colour mixes) to create the CMYK spectrum. For a photograph, or continuous tone image, colour is represented by small printed dots of the four ink colours, which together combine to form a complete image. Via the computer CMYK is simulated on the screen and unless the monitor is calibrated to accurately match this mode, reliable colour matching may be impossible.

The RGB spectrum does not directly convert to CMYK and many colours actually have no direct match. Colours can shift dramatically from one spectrum to the other and images or key colour choices may appear significantly different from the intended effect. Typically converting from RGB to CMYK will ‘muddy’ an image as the intensity of the additive colour on the monitor is replaced by the subtractive colour reflected back to your eye from the printed stock. For CMYK print requirements images converted from RGB must be correctly adjusted to achieve the originally desired colour balance, or workable alternative colours selected.

It is important to understand that supplying a file for print in RGB will likely cause significant colour issues, if printed by an offset press and possible colour shifts if printed digitally. Created files need to be set in CMYK mode and all imported or placed content within the final document needs to be converted to CMYK, if in RGB mode.

Adobe Photoshop, however, offers a different approach for working between colour modes. Certain filters and effects will only operate in RGB mode, so it is possible to create files in RGB as the ‘working’ file and supply a ‘save as’ copy of the final file in CMYK for print purposes. RGB files in Photoshop are also smaller file sizes compared to CMYK files, which is often an advantage when working with the file and also for electronic distribution.

As with converting from one colour mode to another, changing directly to grayscale from either RGB or CMYK without tonal adjustment is not advised. Viewable variances between objects of different colours are often lost if their relative values are similar. As grayscale is a mono (single) colour mode, the amount of information describing image content is limited, compared to a colour mode containing a wider spectrum. Detail that the eye can see through colour differences may now be effectively hidden, once converted to slightly varied tones of a single colour.

Supplying a colour image for grayscale output on a black & white or single colour print project, without the correct mode conversion and correction is a particular area regarding quality concerns. For a desirable printed result, images must be converted to grayscale and the steps illustrated below followed to produce an appropriate file.

The following Adobe Photoshop step-by-step guides provide specific information regarding common problems affecting image quality.

Correcting and improving the appearance of a colour image:

  1. Resize the image to the required output resolution and size.
  2. Select ‘Filter’, then ‘Sharpen’, then ‘Unsharp Mask’. The options here control the amount of contrast Photoshop applies along the edge of areas containing different pixel content. The default values for radius and threshold will be suitable for most images, however experimentation may produce suitable results. The degree of sharpening is determined by the amount percentage. Typically a value in the range of 50-100 will suffice, but for larger images a higher value will be required. Oversharpening can produce a noticeable halo effect, however the effect of this filter are less pronounced when printed.
  3. If required, select ‘Image’, then ‘Adjustments’, then ‘Curves’. Manually adjusting the curve will change the overall tonality and adding points to the curve (by clicking) will allow the adjustment of particular image areas from point to point. You can adjust all or target individual channels.
  4. If required, select ‘Image’, then ‘Adjustments’, then ‘Levels’. The sliders here allow you to compress the shadow and highlight values to match the image content for better output results. You can adjust all or target individual channels. Moving the end sliders below the histogram alters the darkest and lightest pixels within the image and remaps the black or white point to your new value. Adjusting the middle slider affects the gamma (brightness) to make the overall image darker or lighter.
  5. If required, select ‘Image’, then ‘Adjustments’, then ‘Brightness/Contrast’. This is a basic adjustment that affects every pixel in the image. Used sparingly it can apply greater overall contrast than either ‘Curves’ or ‘Levels’, especially on grayscale images.
  6. If required, select ‘Image’, then ‘Adjustments’, then ‘Hue/Saturation’. Under ‘Edit’ you have the option of altering the entire colour space, or individual colour components. Adjusting the values for hue, saturation and lightness can shift the colour through the spectrum and change the colour depth.
  7. If required, select ‘Image’, then ‘Adjustments’, then ‘Color Balance’. Adjusting the values for shadows, midtones or highlights can change a specific tonal range and improve the mixture of colour within focussed areas.
  8. If required, select ‘Image’, then ‘Adjustments’, then ‘Shadow/Highlight’. This corrects the visible lighting of an image. You can individually control the shadows and highlights to brighten detail lost in shadow or correct overexposed elements.


Converting and correcting the appearance of a grayscale image:

  1. Resize the image to the required output resolution and size.
  2. If required, Select ‘Image’, then ‘Mode’, then ‘Grayscale’.
  3. Select ‘Filter’, then ‘Sharpen’, then ‘Unsharp Mask’. The options here control the amount of contrast Photoshop applies along the edge of areas containing different pixel content. The default values for radius and threshold will be suitable for most images, however experimentation may produce suitable results. The degree of sharpening is determined by the amount percentage. Typically a value in the range of 50-100 will suffice, but for larger images a higher value will be required. Oversharpening can produce a noticeable halo effect, however the effect of this filter are less pronounced when printed.
  4. If required, select ‘Image’, then ‘Adjustments’, then ‘Curves’. Manually adjusting the curve will change the overall tonality and adding points to the curve (by clicking) will allow the adjustment of particular image areas from point to point.
  5. If required, select ‘Image’, then ‘Adjustments’, then ‘Levels’. The sliders here allow you to compress the shadow and highlight values to match the image content for better output results. Moving the end sliders below the histogram alters the darkest and lightest pixels within the image and remaps the black or white point to your new value. Adjusting the middle slider affects the gamma (brightness) to make the overall image darker or lighter.
  6. If required, select ‘Image’, then ‘Adjustments’, then ‘Brightness/Contrast’. This is a basic adjustment that affects every pixel in the image. Used sparingly it can apply greater overall contrast than either ‘Curves’ or ‘Levels’, especially on greyscale images.
  7. If required, select ‘Image’, then ‘Adjustments’, then ‘Shadow/Highlight’. This corrects the visible lighting of an image. You can individually control the shadows and highlights to brighten detail lost in shadow or correct overexposed elements.

As a general rule grayscale images usually print well when adjusted to appear slightly too light on the monitor. Darker images can loose a large amount of midtone detail (especially on newsprint or specialty uncoated stocks) and appear filled-in when printed.

Converting from RGB to CMYK:

  1. Select ‘Image’, then ‘Mode’, then ‘CMYK Color’.
  2. In the History palette (if not visible turn on via ‘Window’, then ‘History’) check for any colour shift by moving between the available states, showing the file in both colour modes.
  3. If required, select ‘Image’, then ‘Adjustments’, then ‘Hue/Saturation’. Under ‘Edit’ you have the option of altering the entire colour space, or individual colour components. Adjusting the values for hue, saturation and lightness can correct any shift or loss of colour.
  4. If required, select ‘Image’, then ‘Adjustments’, then ‘Color Balance’. Adjusting the values for shadows, midtones or highlights can correct any colour shift across the entire image and improve the balance within a specific tonal range.
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