Recently a new function appeared in the editor. The U-turn function.
Now the question is, do we enable U-turns (put them on green)?

No, unless making a U-turn is necessary to reach a destination and the road is wide enough to safely make a turn in one smooth motion, or when a U-turn is explicitly marked on the road.

This will be added to the wiki shortly.

NL:

Zetten we u-turns aan (= op groen)?

Nee, tenzij keren nodig is om bestemming aan overzijde van de weg te bereiken en de weg zo breed is dat er veilig in één vloeiende beweging kan worden gekeerd of indien u-turn expliciet voorzien op de weg.