Is colour a purely quantum effect? If the colour of an object is determined by the wave-lengths of light that is absorbs and reflects (?) then can colour be described as a purely quantum effect (i.e. without quantum effects an objects might abosrb all light up to a certain wavelength) or is it more prosaic than that?
 A: No, you can get colours due to purely classical interference, for example in soap films:

(image is from the article linked above).
However the absorption of light is a quantum process, or at least I cannot think of any examples that are not. So if you are generating colour by absorption of light then yes it is a quantum process.
A: I'd take a different tack.  Yes, detection of photons in the retina is quantum.  No, color perception is not.  Our brain does strange things with the relative signal levels coming off the various rods and cones, and reports the perceived colors based on some processing.
The reason I'm suggesting that color perception itself is not quantum, among other things, is that there are many colors which we perceive only as the combination of 2 or more quantum-detected wavelengths.  If you look at a chromaticity diagram, (quoting from the wikipedia page),

The diagram represents all of the chromaticities visible to the
  average person. These are shown in color and this region is called the
  gamut of human vision. The gamut of all visible chromaticities on the
  CIE plot is the tongue-shaped or horseshoe-shaped figure shown in
  color. The curved edge of the gamut is called the spectral locus and
  corresponds to monochromatic light (each point representing a pure hue
  of a single wavelength), with wavelengths listed in nanometers. The
  straight edge on the lower part of the gamut is called the line of
  purples. These colors, although they are on the border of the gamut,
  have no counterpart in monochromatic light. Less saturated colors
  appear in the interior of the figure with white at the center.

A: Herewith a few notes you might find helpful:
Physicists talk about two kinds of fields: classical fields and quantum fields. Actually, we believe that all fields in nature are quantum fields. A classical field is just a special large-scale manifestation of a quantum field. (My emphasis.)
~Dyson
If you ask a physicist what is his idea of yellow light, he will tell you that it is transversal electromagnetic waves of wavelength in the neighborhood of 590 millimicrons. If you ask him: But where does yellow come in? he will say: In my picture not at all, but these kinds of vibrations, when they hit the retina of a healthy eye, give the person whose eye it is the sensation of yellow.
~Schrödinger
Thus the colors with their various qualities and intensities fulfill  the axioms of vector geometry if addition is interpreted as mixing; consequently, projective geometry applies to the color qualities.
~Weyl
It is just like the mathematics of the addition of vectors, where (a, b, c) are the components of one vector, and (a', b', c') are those of another vector, and the new light Z is then the "sum" of the vectors. This subject has always appealed to physicists and mathematicians. In fact, Schrödinger wrote a wonderful paper on color vision in which he developed this theory of vector analysis as applied to the mixing of colors.
~Feynman
The mathematical machinery of QM became that of spectral analysis.
~Steen
I have concentrated on examples from physics since they can be described independently of mathematics. These examples can also be formulated in precise mathematical terms. In this way, they become part of harmonic analysis in mathematics, interpreted in the broad sense above. The general duality between geometric objects and spectral objects permeates mathematics. It has a pervasive influence in such diverse areas as group theory, topology, differential geometry, number theory and algebraic geometry, as well as the areas of analysis and differential equations with which it is more traditionally associated.
~Arthur
