Fun trivia: Feu Vert is the name of a popular french auto parts brand
Less fun trivia: When you pass the Feu Rouge and the police see you, you get to pay lots of money
Would "the fire is green" work as well? (because this is something that exists and could be said)
Well, it doesn't accept that now. I was thinking of the green fire you get when burning copper.
"The fire is green" was accepted on the 18th of September 2017. Like m_jepson, I was thinking of happy days in the science lab, throwing chemicals into bunsen burner flames and writing down what colours they turned the flame.
It's accepted now. Btw there's a less commonly known metal that gives green flame as well, named barium.
I knew something was wrong when I saw "the fire is green". Good thing I checked the tanslation. Never learned "feu" to mean light.
Traffic lights are called "les feux de signalisation " or "les feux tricolores" - no alternative with "lumières".
I am guessing that "lumière" is something that gives light or sheds light or is used to illuminate something, whereas "le feu" is usually used for some type of signal or indicator light. http://www.larousse.com/en/dictionaries/french-english/feu/33379
In South Africa, Zimbabwe and probably other Southern African countries, traffic lights are commonly called robots. Since I'm old enough to remember when traffic lights were first installed in the cities they replaced policemen who used to direct the traffic at intersections. One day my Dad told me that we would soon have robots doing that. It was a bit of a letdown when the robots turned out to be an array of different coloured lights.
I was discussing this point with a friend from Mauritius some years ago and he assured me that there they were called 'robots' but with the French pronunciation of course.
I tried 'robot' in my answer here knowing that I'd be marked wrong, but commented that it should have been marked correct.
Sorry, as a French person, I have never heard anyone using "des robots" to refer to "les feux de signalisation" (traffic lights).