Dusk

In this picture, While I was playing Pokemon go, I happen to be walk past this and I just had to take picture of this beautiful Sunset. Sometimes you don't have to look at the time when you can already tell if it's going to be night time by looking at the sunset. Why does the sky have primary colours? Why can't the sky have colours like green or purple? Actually the sky might have those colours, like the aurora northern lights. I have never seen the aurora sky before but I would love to travel someday just to see it.