The law of supply and demand explains how changes in a product's market price relate to its supply and demand. Demand for basic necessities is less responsive.
interesting that the gold is just a little more expensive - as they are revolting to look at, one wonders whether this is to give them an air of being a little special!
Currys and Amazon are both selling at the same price as the ones mentioned from John Lewis so most likely someone changed their price and the others followed to price match and not lose custom.
I remember seeing price differences depending on colour when I was purchasing my iPad Air. The space grey was the cheapest by around £20 which I was chuffed with as it was the colour I wanted anyway. No real reason for a price difference apart from the retailer deciding it based on demand.