Landing in LHR this morning I noticed that a number of planes had green dye on their tails. Maybe because I'm usually sat in an aisle seat I hadn't notice this before. I'm presuming this is just de-icer. The only thing that confused me was a number of these planes still had the airbridge connected and my limited experience from the US was planes were de-iced once doors was shut and airbridge disconnected.

My only other thought was whether they were marking planes once they had installed wifi into them - but I'm guessing that more wishful thinking.

Any insights from those in the know?