/jp/ - Otaku Culture

Just started reading yotsuba and one thing confused me in the last chapter of the first volume. Yotsuba points at the rain clouds and calls them green. Is that an idiom or a cultural thing kinda like calling the green traffic light blue or is that just yotsuba being yotsuba? I tried googling it but I couldn't find an explanation, and I even looked up the official translation for it and it also just said "the clouds are green".