Sets the colour table for 2D pixel plots. In default mode
it prompts for an external table to read in. This may be an
HDS object of the right type and size, or the number of a
standard ASTERIX colour table. There are two other modes
invoked by including one of the following keywords: NEG produces
the negative of the present table (note that using NEG again does
not get back to the original table - use NONEG); RGB produces a
special 3-colour table for use with images created by MKCOLIM.