"The bathroom's lamp is green."
Translation:La lampada del bagno è verde.
I asked an Italian friend and the best explanation was di is for possession i.e. "la lampada di Maria" but del is used for "of the/in the". As the bathroom can't own the lamp (the lamp is is merely in the bathroom), del is used rather than di. I'm not sure if this is 100% correct but that is how it was explained to me by a native speaker :-)