The classic ones are red, there are (were) ones installed in the 80s-2000s which are mostly glass and grey aluminium (IIRC) that had a the BT logo on the side.

The classic red ones are more typically associated with smaller villages etc nowadays, often turned into mini community libraries or have been retrofitted to carry defibs. Some towns still have them but very few still function.

Also for context about the original picture, I believe it's a reclamation yard type of thing, you can often buy old/vintage building materials/farm equipment/utilities etc at them for a somewhat inflated price

They're red until they moult. Then they turn blue.