Colour adjustments

Hans Weichselbaum walks us through correcting an image with problematic colours

Getting colours right is one of the main challenges in digital imaging. Concerning hardware — cameras, monitors and printers — we have seen huge improvements in recent years. Longevity of inks and fading colours, major challenges just a couple of years ago, are not big issues anymore. But still, we often come across images that don’t seem right, and in many cases it is just a matter of adjusting the colour balance. Perhaps the Auto White Balance of your camera didn’t catch the scene properly, or you have a scan of an old photo with faded colours. You might also try to warm the colours of a portrait shot or reduce the red cast of a scene taken in artificial light.

A few months ago we looked at colour management. This is always the first step, making sure that monitor and printer give you an accurate colour representation of the image pixels. Only when you are working with a calibrated monitor and the right printer profiles does it make sense to fine-tune the colours.

First, we’ll look at the White Balance — that’s where it all starts. Then we’ll go through some of the common tools for tweaking colours, which you can find in every digital imaging program. Finally, I’ll introduce you to two methods that can be quite handy. They allow you to snap an image straight into the optimum colour balance. But let’s start by looking at the theory behind colour adjustment.

The colour wheel

The visible region of light covers the electromagnetic spectrum from 380nm (violet) to 700nm (deep red). If we wrap this around a circle we get the colour wheel (left).

Red, green and blue (RGB) are the primary additive colours, because adding them up gives us white. In-between we find the primary subtractive colours, cyan, magenta and yellow (CMY). These are the colour inks we use for printing and they add up to black. It is important to see the relationship between all of these colours. Once you understand the links, then getting rid of a colour cast becomes much easier. For example, if the image has a blue cast, you cannot simply take blue out. You have to shift the colour balance from blue to the opposite colour, which is yellow. In general, all colour adjustment tools give you three controls that adjust the balance between red-cyan, green-magenta and blue-yellow.

White Balance — It all starts in the camera

In most cases the Auto White Balance (Auto WB) setting of your camera will do a good job. If not, the colours need to be corrected later in Photoshop. Auto WB works similarly to the human eye: The lightest object in the scene is set to white, or light grey. For example, in a landscape the clouds will be the lightest objects. They are a good indication of the light temperature, and the lightest part of a cloud should come out white or light grey.

If you shoot in JPEG then it is important to get the WB right. A daylight scene will have a strong blue cast if the camera was set to tungsten light. Such a colour cast is difficult to get rid of later when you work in RGB. If you don’t want to fiddle around with the WB, then it is best to leave the camera in the Auto WB setting.

On the other hand, if you shoot in Raw, the camera settings are not critical and you have total control over the WB setting during Raw conversion. The image on the right shows you the WB setting in Lightroom (Adobe Camera Raw, and all the other Raw converters, give you the same controls).

The Eyedropper on the left works as an auto setting when you use it on a neutral light area of your image, for example, a cloud. Or you might prefer the finer control you get with the two sliders, the top one for the Colour Temperature scale and the second for the Tint.

Colour Temperature is measured in Kelvin (K), with 0K being the absolute zero temperature (-273.16°C). The tungsten filament in an ordinary light bulb glows at about 2800K, and that’s the colour temperature of a light bulb. The surface temperature of the sun is approximately 5500K and that represents our ‘daylight’ reference number. Light is scattered in the atmosphere, which gives us the blue sky, and this is the reason why the colour temperature keeps going up under overcast conditions.

The image on the left shows you the colour temperature scale from candlelight (below 2000K) going up to very high values under blue sky.

The industrial reference value of D50 is not exactly the same as ‘daylight 5000K’, but it is close enough for us to ignore any differences. It is the standard light temperature for professional light tables and viewing booths. A photo print should always be viewed and judged under a 5000K light source. On the other hand, monitors are calibrated to the D65 standard (6500K).

Note that blue is a ‘hot’ colour, while red is a rather ‘cool’ colour. This is the opposite of our traditional thinking, but it does make sense if you think of heating a piece of metal. It will start by glowing dark red, then go through orange, yellow and white. And if a light bulb blows out, it will give a distinct bluish flash.

Coming back to the White Balance setting in our Raw converter, the Temperature control specifies the colour temperature in Kelvin on an amber-blue scale. Lowering the colour temperature makes the image bluer to compensate for the amber light; raising the colour temperature makes the image more amber, to compensate for the bluer light.

The second slider, the Tint control, lets you fine-tune the colour balance along the green-magenta axis, perpendicular to the Temperature control. Negative values add green, positive ones add magenta.

It takes a while to get your head around these controls, because we are used to three sliders in RGB: red-cyan, green-magenta and blue-yellow. Keep in mind that Lightroom, as well as Adobe Camera Raw, also work with JPEG, TIFF and PSD image files, but in general you will want to use the standard colour adjustment tools for fine-tuning the colours in an image.

The common colour adjustment tools

There are many ways of changing the colour balance. The most common tool is the Colour Balance interface (Image>Adjustments>Colour Balance) shown in the above image.

Here you find the three sliders we were talking about earlier for adjusting the balance between the primary colours. The interface allows you to do this separately for shadows, midtones and highlights. For example, to eliminate a blue cast in the shadows you would select Shadows under Tone Balance and shift the Blue slider into negative territory (less blue, more yellow).

You can also use the Levels command for colour adjustment. There is a drop-down menu called Channel with a default setting of RGB. The interface allows you to select an individual channel, e.g. the Red channel. You can then shift the red-cyan balance using the middle slider.

The same goes for the Curves command. Similar to Levels, you can make adjustments to the combined RGB channels or to individual colour channels. In the example shown to the right we have the Blue channel selected and we get rid of a blue cast by grabbing the curve and pulling it down (reducing blue and enhancing yellow). This is like using the middle slider in the Levels command.

But there is more you can do in Curves, for example, if you have a green cast in the shadows and a magenta cast in the highlights, you simply select the Green channel and make an S-curve (increase magenta in the shadows and reduce it in the highlights, without affecting midtones).

Another handy tool is called Variations (Image>Adjustments>Variations) shown on the left. The beginner, who often doesn’t know which way to adjust, will especially feel comfortable with this interface. It shows you the original photo surrounded by six images with the colour balance shifted towards one of the six primary colours. You simply click on the one that looks best and it will be placed into the middle. Then you can further fine-tune the colours, if necessary.

Two more ways of eliminating colour casts

When looking at a photograph you often feel that the colours are not right, but it is not obvious where the problem lies. In that case you might try one of the two following methods.

The first one uses the Match Colour command in Photoshop. This comes with an advanced algorithm to match colours and brightness between two images, but you can also use it on a single image.

Open the image and duplicate the layer by dragging the Background Layer over the New Layer button in the Layer palette (it is always a good idea to work on a duplicate layer). Then open the Match Colour command (Image>Adjustments>Match Colour), shown above:

Click on Neutralise to remove the colour cast. If the effect is too strong, use the Fade slider. Also try the Colour Intensity slider to increase the colour range, if necessary. Stay away from the Luminance slider – there are better ways of optimising the lightness (use Levels or Curves instead). You can also change the opacity of the layer, or even create a Layer Mask to limit the colour changes to isolated areas.

The second method uses the Average Blur filter, which was introduced in Photoshop CS. Again, start by making a duplicate layer, then go to Filter>Blur>Average. You will get a layer with a solid colour, the average colour of your image. This will be the problem colour. To get rid of it, you need to invert this colour (Image> Adjustments> Invert) to apply the opposite to our image. Finally, you change the Blending Mode of the top layer to Colour. If the compensating effect is too strong, you can tone it down by reducing the Layer Opacity, shown right.

These two methods assume a neutral colour balance of your photo. They won’t work if there is a strong imbalance, for example with sunset images, which naturally have a strong orange cast.