Modified Colour class API to always use "colour", not "color"

The Colour class includes a number of methods and internal variables
using the spelling "color". We ought to standardise on one spelling
and use that exclusively, and that spelling ideally ought to be the
English one (to match the class name and documentation).
Merge pull request #2 from simpleigh/english_colours

Modified Colour class API to always use "colour", not "color"