This tutorial will introduce you to a method for correcting colour and contrast within your image via Levels. Before you start, I’d recommend you take a look at my ‘Photoshop Levels – Fixing Contrast – Black & White‘ tutorial to establish a good foundation knowledge of Levels.
Levels: Colour Channels
A channel is a ‘plate’ of tonal information that represents the strength of a particular colour, i.e. ‘Red’ (in ‘R’GB). This system is also used for print worthy CMYK and other color modes. A channel in itself is little more than a grayscale image (0-255 with 256 individual shades). RGB and CMYK colour images have multiple channels, combining to form composites hosting millions of visible hues and shades.
RGB: Red, Green, Blue. Used for screen output, i.e. web use, on-screen presentations, video, etc. Up to 16.7 million possible hues and shades (256*256*256=16,777,216 colours).
CMYK: Cyan, Yellow, Magenta, Black (or Key). Used to print. This time an additional channel (256*256*256*256 = 4,294 ,967 ,296 colours)
For more information on channels, please read ‘Photoshop: Color Mode | RGB – Grayscale – CMYK‘.
In the following example, I’ll be using a discoloured and dimly lit RGB image. The following principle can be applied to a CMYK image in exacting the same way.
As you can see from my image, there is a lot of room for improvement. Your photo might not be as extreme as this, but the following exercise can be applied to the vast majority of colour digital images.
Step 1: Calling up Levels
Call up the Levels dialogue box (menu) Image->Adjustments->Levels or use the quick keys CTRL+L (recommended method). The Levels dialogue box should now appear (fig.1). You can also apply levels as part of an Adjustment Layer.
Step 2: Selecting a Channel
At the top left of the Levels dialogue window, you should find the Channels drop-down menu. By default, it will be set to the composite (RGB in this example). Select each and apply the following to each Channel in turn.
In the centre of the dialogue box you’ll see a histogram, representing the information available within that channel across the tonal range.
Along the bottom of the histogram are three pointers, black, gray and white. Slide the black and white pointers inward until they start to touch the information within the histogram. You can later adjust the gray pointer if your image is a little dark or discoloured after performing levels.
As you drag these pointers inward, the overall contrast will increase. If you need to decrease an already contrast-heavy image, then use the two other sliders (black and white) on the bar beneath the histogram. These sliders will have the opposite effect.
Now repeat this process within the remaining Channels.
From the Channel drop-down, select ‘Red’. Now perform the same adjustment principles as you would with a grey-scale image – see ‘Photoshop Levels – Fixing Contrast – Black & White‘. (Fig.2)
Step 3: Repeat process through all other channels
Now repeat this process for the remaining Channels.
Step 4: Colour tweaking
Your image should now looked much improved. If the contrast is now fixed, but the colours are a little out, then try this:
My example image had a purple cast. This demonstrates that the blue and the red channels where more dominate than the green.
To fix this, open the Levels dialogue box CTRL+L and select the weaker channel (in this case Green). Slide the mid point (gray) slide immediately beneath the histogram to the left to increase the intensity. Sliding to the right will decrease the intensity.