The art of colour grading

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A red orange and some black gold? SFX professional Amitaabh Naaraayan explains some of the nuances of colour grading

Colour grading is much more than just making colours match from shot to shot — it’s also a way of selectively changing the look and feel of a clip. In their most basic form, colour correction tools enable you to make your whites look white and your blacks look black, so that everything appears ‘normal’.

If you shoot a sequence over an hour as the sun’s going down with changes in colour temperature, you can make everything look like it was shot in one go. Colour grading can also be a creative tool, and digital colour grading has seen the process pushed to exciting new limits over recent years.

Films such as Slumdog Millionaire, The Lord of the Rings and television series like Lost, Heroes, Prison Break, have all used deep colour grading to create different moods, or to let the viewer know what part of the story’s world they are in.

Altering the colour content of your footage to create a colour look across a series of shots, scenes, or the entire project is the primary objective of colour grading (also referred to as colour timing).

In order to understand colour grading, it is essential to understand logarithmic and linear modes. Film naturally captures light as logarithmic, but these terms are usually used to describe two different ways of digitally scanning an image. Most commonly, the term is used in scanning film, and in creating digital intermediates to go back to film. Simply put, linear capture means each digital step, from dark to light is equal. A linear sequence is 1, 2, 3, 4, 5, 6, 7, 8.... For logarithmic capture each step is double the previous. A log sequence is 1, 2, 4, 8, 16, 32, 64, 128... If the total number of steps is kept the same, log captures a much greater dynamic range than linear, but it looks very flat. It is therefore ideal as a digital intermediate format. The mode you select defines the colour space, your work environment, and the availability of some hot keys.

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One of the first steps in the colour grading process is the creation of a Continuity Grade — a consistent colour look across a series of shots and over time. Continuity grades convey the time frame and should be invisible to the viewer. Generally, continuity grades are created when you balance the colours in the shots during the initial primary colour grade.

Primary Colour Grading is a concept applied to the entire image and is used to obtain an overall colour look for each shot used in a series of shots, scene, or entire project. When you perform primary colour grading, you modify the brightness and contrast of the red, green, and blue channels independently. One can modify the red, green, and blue channels together (with the Brightness and Contrast sliders); however, the channels are not co-dependent — a change made to the red channel does not depend on a change made to the green or blue channel.

Primary Colour Grading is performed at the beginning and end of the colour grading process. In Log mode, you modify brightness, contrast, and saturation across the entire image, as well as brightness in the shadows, midtones, and highlights. In Linear mode, you modify lift, gain, gamma, saturation, and contrast for the red, green, and blue channels across the entire image or in the shadows, midtones, and highlights.

Note: The intensity of specific colour channels (red, green, and blue) determines whether the pixel is part of the image shadows, midtones, or highlights.

Curves Colour Grading is used to further modify the RGB and Hue curves. Curves colour grading is well suited, for example, for colour grading a specific range of colours without having to pull a key. The RGB curves can be altered to remap red, green and blue values either simultaneously or individually. Use the Hue curves to perform hue shifts, lighten or darken colour ranges, and saturate or suppress colour or luminance ranges. You can also plot colours and add vertices for increased precision.

Secondary Colour Grading is used to colour grade specific hues and areas in an image. Create secondary’s by generating keys and geometries. You can combine keys and geometries to define the area for modification—keys to define a colour or range of colour and geometries to define an area. After areas have been defined, you can colour grade them using the Grading and Curves tools. Objects can be tracked within the image to animate a geometry used for a secondary colour grade.

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