adjectives + narimasu

Question

Good evening,

I've been studying Japanese here in Tokyo for about a month now in a Nihongo Gakuin. One of the lessons that we encountered was adding adjectives + narimasu like for example:

osoi = osoku narimasu
ookii = ookiku narimasu
hayai = hayaku narimasu

From what I understood it's changing the last i = ku in i-adjectives so it becomes a noun, or simply adding ~ ni to na-adjectives. Although, I was wondering how that would apply to colors. For example, akai. Does it become akaku narimasu or aka ni narimasu?

しんごうはあかくなりました？ 
しんごうはあかになりました？

The traffic light became red. Any help is greatly appreciated. Thank you very much!

Answer 1

In most contexts, the most natural way to say something “turns red” or “becomes red” is by using あかくなります. For example, わたしのかおがあかくなりました would be one way to say “my face turned red,” and you can also say そらがあかくなりました (“the sky turned red”). The same pattern holds for other color adjectives that end in い (e.g., そらがあおくなりました - "the sky turned blue").

However, for the particular example you’ve asked about – a traffic light – it would be much more natural to use the noun form + に. I suspect that this is because traffic lights don’t actually “turn red” at all, at least in the sense that a face or a sky can turn red – what really happens is that an amber light goes off, and a red light goes on. (Maybe they are really plain lightbulbs, one behind an amber lens and one behind a red lens, but you get the idea.) It may also be relevant that the Japanese term doesn't conflate the entire apparatus with the individual light(s) that are part of it, as the English term "traffic light" does. (しんごう is really closer to "traffic signal.") In any case, it is far more usual to say しんごうがあかになりました.