Actually, none of the above. The air doesn't have a color (unless you live in LA). That's why you can see stars at night. The reason it looks blue during the day is due to the reflection of light as certain parts of the light spectrum are reflected at angles while blue isn't. Thus, the sky is not blue. Of course, since you used the UK version of colour vs color, it is entirely possible that you are asking because you have never actually seen the sky due to cloud cover. So, if that is the case, the sky can be whatever colour you wish it to be. Just keep dreaming.






