The reason the sky turns pink is because of sunlight that passes through the sky.
As that sunlight passes through the sky the sunlight passes through evaporated water and gas and clouds and other particles in the air.
When the sunlight passes through evaporated water and gas and clouds and other particles in the air the particles then refract and reflect the light, scattering some of sunlight's colors which in turn makes the sky turn pink.
The sky can turn many different colors depending on what colors are passing through the water and gas and clouds or any other particles.
Sometimes the sky can look blue, sometimes the sky can look red, pink or any other colors.
Although most times we mostly see the sky turn pink.