"My brother has a green thumb."
Translation:Broren min har grønne fingre.
Does it mean he is good with plants or that his thumb is green? Because I believe grønne fingre is not limited to the thumb...
Grønne fingre is literally "green fingers", as you suspect. And it also means he is good with plants.
I am not really sure how this sentence works. I mean, isn't fingre plural form of en finger? Which would mean that the sentence is incorrectly translated? Because a thumb in norwegian is en tommel. So shouldn't the whole sentence go more like "Broren min har en grønn tommel"?
It seems that American English has the idiom: "a green thumb" and Norwegian has a similar idiom: "green fingers" and both mean that one is skilled at growing plants. Translating from one idiom to another will not always be a word for word translation.