If you really want light-sensitive concrete slabs in different colours, why not B&Q it?
I had to check the date was Friday 13th, not April 1st! I may be missing the point, but couldn't Daniel Edwards (AJ 12.6.03) have saved himself considerable time and effort (not to mention the rain forests - six pages! ) had he made a trip to his local B&Q and purchased two no. 3'x2' concrete slabs, which are also 'highly sensitive to changes in the light'.
He may even be able to select some slabs that vary in colour.
I'm sure B&Q's highly trained staff would have been able to assist in the selection process from the shop's wide range. The lighting department could also have provided some fittings capable of 'ethereal light projection' with which to illuminate these 'works of art/paving slabs'.
Douglas McKirdy, armarchitect , Glasgow