Provides operations and type testing for Sass colours.
Colour operations are all piecewise, e.g. when adding two colours each
component is added independantly; Rr = R1 + R2, Gr = G1 + G2, Br = B1 + B2.
Resulting colours are returned as a named colour if possible or #rrggbb.
Transparent is a SassColour. Operations on transparent return transparent.