Thanks Trev! I’ve got a feeling in a Renault I’ve seen green → amber → blue → red. It was something odd as I remember at the time thinking it didn’t make any sense to me. Maybe there isn’t a standard then by the sounds of it. I never really thought of it too much since then but am revising for a driver assessment here - mocktheorytest.com/highway-code/heavy_vehicle/ - and a question came up on the colours and it got me wondering again…
A couple of times a year I take my car very near to the red line in third on a quiet stretch of dual carriageway. I put some engine cleaner in the fuel first. A whole load of crap comes out the back. The engine seems happier after tho. But I won’t be trying it in a truck 