This attribute determines the colour index used when drawing each
element of of a plot. It takes a separate value for each graphical
element so that, for instance, the setting "Colour(title)=2" causes
the title to be drawn using colour index 2. Standard X colour
names, space-separated floating-point RGB triples, or
HTML colour codes
(optionally using a "@" inplace of a "#" to avoid the code
being interpreted as a comment within a style file) can also be
specified, and the nearest available colour will be used if
the requested colour is not currently in the palette.
The default behaviour is for all graphical elements to be drawn
using Pen 1.
For a list of the graphical elements available, see
If no graphical element is specified, (e.g.Colour instead
of Colour(title)), then all graphical elements are affected.