why is the sky blue in colour?

There are many different colours of the sky - some white, some blue and others grey. The colour we typically recognise as blue is called azure. On a clear day, we can see the entire spectrum of colours in the sky and this depends on many factors such as pollution and atmospheric conditions. However, one major role in determining what your perceive to be "blue" is an afterimage which has been created by whiteness right before your eyes. This effect is caused by the light-sensitive cells in our eye seeing light reflecting off near objects before fading into blackness - our brain then concludes that this object must be white because it was just seen with such clarity that it would not be possible if it was any other colour.