Does anybody have any information about the Grey Coat that the Foot Guards wear sometimes?

I purchased a replica of a Grenadier Guards British Army Cap Badge and when I was looking for some information about the regiment on the web I came across this picture of a rare uniform I had never seen before: A regiment of Coldstream/Grenadier guards wearing a long grey coat. Does anybody know if they wear it in "special ocations" or is just a variation of the traditional red coat?

I'm not British so I don't know much about British Army.