1. A method of modifying the appearance attributes of a stream of digital video images comprising: on a first occasion, passing the stream of digital video images through a colour corrector in a single channel, viewing the video images and defining a first set of appearance attribute corrections in a first region of colour space to be applied to the images, determining a first set of appearance attribute correction parameters corresponding to the first set of appearance attribute corrections, storing the first set of appearance attribute correction parameters, and generating a first lookup table for implementing said first set of appearance attribute corrections; on a second occasion, passing the stream of digital video images through said colour corrector in said single channel, viewing the video images, and defining a second set of appearance attribute corrections in a second region of colour space to be applied to the images, determining a second set of appearance attribute correction parameters corresponding to the second set of appearance attribute corrections, storing the second set of appearance attribute correction parameters with the first set of appearance attribute correction parameters in a combined single set of appearance attribute correction parameters, generating a second lookup table for implementing said second set of appearance attribute corrections, and merging the first and second lookup tables to generate a cumulative lookup table for implementing the combination of said first and said second sets of appearance attribute corrections; and on a third occasion, passing the stream of digital video images through said colour corrector in said single channel and applying the single set of appearance attribute correction parameters to the video images so as to implement said first and second sets of appearance attribute corrections to the video images.