Maybe they just went from the hottest colour into the coldest colour (づ￣ ³￣)づ
Could be that red is the color of violence, war, and anger or evil, while blue is calming, trust, righteousness, and peace. No idea.
Most likely... They are often seen as opposing colours (not literally, but because of the meaning behind the two colours and how they contrast)
I guess it makes sense, thanks.
I know it's not that important but... the mangaka is an artist, how does she not know that the opposite color of red is not blue but green?? That's very basic knowledge. I hope it's just a mistake in translation.