[donretag]

I assumed it had something to do with the color and tried to find patterns. Turns out you need knowledge of the London Metro system. It would be nice if the title reflected that.

[TRiG_Ireland]

Well, the title says "tube", which would be enough of a hint for most people, I think.

[NBJack]

Please define "most people".

[TRiG_Ireland]

Well, I suppose my sample is mostly "public transport nerds", which may not be representative.

[Fatnino]

People who know that in london the subway is called the tube.