Most image editors include tools for adjusting the hue and intensity of colours, as well as adjusting the overall colour balance in images. These adjustments are necessary because image sensors sometimes fail to record ‘true’ colours or produce colours that are out-of-balance in one or more ways.
Colour balance adjustments are normally used to eliminate undesirable colour casts in images, although, because most tools act across the entire image, some can also be used to emphasise particular hues. Most colour casts appear because image sensors are designed primarily for recording images in daylight.
Different types of lighting can produce colour casts, which are usually corrected by the camera’s auto white balance control. Problems occur when different light colours, such as the incandescent and daylight lighting shown in this photograph, occur in the same image. The white balance control has adjusted for the indoor lighting (which covers most of the image), creating a blue cast in the areas lit by daylight. The only way to eliminate this cast would be by selecting the day lit areas and applying corrections to those areas alone.
Taking photos under artificial light usually shifts the colour balance, sometimes dramatically ““ as under incandescent lighting. Quite distinct colour casts can also be introduced at different times of day (particularly at sunset) and light can have a subtly different colour balance at different latitudes and under different environmental conditions. So, although your camera’s white balance control will address some of these issues, when you need precise colour control, the only way to achieve it is through editing.
Auto colour adjustment
Many image editors provide an automatic colour adjustment function, although it may not bear that name and it may be included in one of the quick fix modes. For images where the colour balance is close to the natural daylight balance, the auto correction modes will generally do a good job and many will correct exposures and contrast at the same time.
The Auto Colour setting in Photoshop usually provides excellent, one-click corrections for images with a normal daylight colour balance.
Problems often arise with images taken when the ambient light has a different colour balance that you want to retain; for example shots taken near sunrise or sunset, where the warm colours are an important part of the picture. The example below shows how the auto colour adjustment will strip the warm cast from an evening shot, thereby removing most of the image’s impact.
When faced with this problem, either reduce the auto adjustment with the Fade slider or cancel the auto adjustment.
The original image (top) shown with the same image (below) after Auto Colour adjustment in Photoshop.
Colour correction with Levels
Many photographers don’t realise the Levels adjustment can also be used to correct the colour balance in images. Instead of simply using the default RGB setting in the Channel drop-down menu, you can make separate adjustments to the Red, Green and Blue channels. This tweaks both the tonal range and colour balance and will save you time when your originals are only slightly off the mark.
The default setting for the combined RGB channels in the Levels doalog box with the end points of the histogram circled in red.
Upon opening the Levels dialog box, select the red channel and hold down the Alt/Option key on your computer’s keyboard. When you begin to move the white point arrow inwards the image will change quickly to total black. As the arrow is moved, red pixels will start to appear.
Holding down the Alt/Option key and moving the white point arrow inwards sets the limits for the highlights.
At this point, stop moving the white point arrow and change to the black point arrow on the left. Holding down the Alt/Option key changes the image to red. As you slowly move the black point inwards, black pixels will start to appear, enabling you to set the maximum shadow density. Stop as soon as you see a small cluster of black points to prevent a loss of shadow details.
Use the black point arrow to set the image’s shadow density.
Repeat the process for each colour channel, taking note of the different shapes of the histogram for each channel. In each case, the image will turn green when black point slider is selected in the green channel mode and blue in the blue channel mode.
The middle arrow can be used to fine-tune the balance of the colour channel you are adjusting. For the red channel, moving it to the right pushes the colour balance towards cyan, while moving left goes towards red. With the green channel, moving the middle arrow to the right pushes the colour towards magenta, while moving left shifts it towards green. The middle arrow on the blue channel makes the image more yellow when it’s moved to the right and more blue when moved to the left.
Moving the middle arrow on the blue channel slider (circled) to the right shifts the colour balance towards yellow.
You can return to the RGB channel and adjust the middle arrow to correct the mid tone brightness in the image globally, without altering the colour balance.
Colour intensity adjustments
Many digital images appear at first to be lacking the intensity of the scene you remember, another native flaw in image sensors. Manufacturers of snapshooters’ cameras commonly boost the colour saturation artificially to provide satisfyingly rich colours. The JPEGs from some of the more serious cameras may also have boosted saturation.
The term ‘saturation’ refers to the purity and intensity of a hue. Vivid primary colours are highly saturated, while pastel colours are unsaturated. Monochrome (black and white) images are totally de-saturated since they only contain black, white and grey.
The effect of saturation adjustments can be seen in these three images. The original picture straight from the camera is in the centre, with one showing reduced saturation above it and one with increased saturation below it.
Moving the saturation slider to the right will uniformly increase the intensity of all hues in the image. If any of those colours is already quite saturated, pushing it up to its limit will cause clipping, which results in a loss of detail in the clipped area.
Too much saturation can present serious problems in portraits because it will cause skin tones to appear bright orange and unnatural. So care is needed whenever you adjust saturation.
Adobe’s editing programs (Photoshop, Photoshop Elements and Lightroom) include a ‘Vibrance’ tool, which works like a saturation adjustment but selectively increases the saturation of less-saturated colours to prevent any hues from becoming over-saturated. Using this tool can prevent skin tones from appearing unnatural while giving colours a ‘lift’.
These images illustrate the difference between the saturation and vibrance adjustments.
You can also choose to go the other way and reduce the intensity of colours, a strategy that applies particularly well when you want to soften and add a dreamy feel to a photo. The sponge tool, which is one of the dodging/burning tool options, can be used as a saturation brush to desaturate or saturate specific areas in an image. Select a medium, soft-edge brush from the Brush pop-up palette and choose Mode>Saturate to increase saturation.
This illustration shows how the sponge tool can be used as a saturation brush to selectively increase saturation in the clouds without changing the intensity of colours elsewhere in the image. The image on the left is the original image from the camera.
Further control over the effect of the brush is available through the Flow setting. Entering 50% gives a medium intensity level when the 100% setting applies a dramatic adjustment.
The eyedropper icon on an editor’s toolbar indicates the colour picker. It is used to sample pixel colour values from anywhere in the image and can provide an easy way to pick up the colour of the sky when you want to add a gradient to darken the sky from the top down.
Sampling colours with the colour picker (eyedropper tool). Clicking on the Foreground/Background icon (circled in red) opens a dialog box that shows you the colour on a hue and lightness spread and lets you shift to a more saturated, darker version of the colour, which can be used for the gradient fill (shown below).
When working with the brush, pencil, type, gradient or bucket tools holding down the Option/Alt key will temporarily access the eyedropper tool. Clicking with the Option/Alt key held down, will sample the selected colour and make it the new foreground colour.
The sample size dropdown menu lets you choose the size of the sampled area for the colour picker. Larger samples will provide greater colour accuracy while small samples ensure precise colour selection.
The colour picker tool can also be used for sampling an image colour to help you choose a similar or complementary colour for text or graphics that will be combined with an image. The foreground/background palette in the toolbar has a double-ended arrow that lets you switch between colours set for the foreground and background.
Photoshop uses the foreground colour to paint, fill and stroke selections and the background colour to make gradient fills and fill in the erased areas of an image. the foreground and background colours are also used by some special effects filters.
Colour space settings
Advanced digital cameras usually provide two colour space options: sRGB and Adobe RGB. Each has a pre-determined way of organising colours to ensure they are reproduced in the same way on different output devices, whether these be computer or portable device screens or in hard-copy prints.
The sRGB is the ‘universal’ colour space used by all devices. While it only encompasses about 35% of all visible colours, it is broad enough for most applications and is the standard colour space for displaying images on the internet. Adobe RGB encompasses roughly 50% of all visible colours and provides better reproduction of cyan-greens than sRGB. It can reproduce most of the colours achievable on digital printers, including inkjets and photolab printers.
Most hobbyist photographers will find sRGB ‘good enough’ for their requirements, particularly if they shoot JPEGs and share images mainly via online applications. Serious photographers who print their photographs will be better served by Adobe RGB because of its wider colour gamut in the greenish-blue part of the spectrum.
For optimum results, the colour space in the image should match that of the output device. So if you plan to print your photographs, you should select the Adobe RGB colour space in your camera and use it in your image editor. Use sRGB for shots that will be viewed on screens.
These two graphs, captured while calibrating a monitor, show the differences between the sRGB and Adobe RGB colour spaces. The sRGB colour space is contained within the green triangle on the left side; the Adobe RGB colour space fits within the purple triangle on the right side. The red triangle represents the monitor’s colour gamut (all the colours it can display). You can easily see the Adobe RGB colour space extends much more into the green part of the visible spectrum (which, coincidentally, is where human eyes are most sensitive and discerning).
Excerpt from Photo Editing Pocket Guide