Question

why is the colour of sky blue

Answer 1

The scattering of blue component of the white sunlight by air molecules present in the atmosphere causes the blue colour of the sky

Answer 2

The sunlight consists of 7 colors namely violet, indigo, blue, green, yellow, orange and red.
In these colors of light the violet or blue because of their short wavelength, they scatter all over the sky more than other colors of light. So, the sky appears blue.