At noon the sun is right above our heads, and sunlight passes through a thinner atmosphere. As the optical wavelength of blue light is shorter, we see the blue part of the sunlight scattered, making the sky appear blue.
At sunset, the sunlight passes through a thicker atmosphere, and blue light has finished scattering halfway through, it is then the turn of orange and red light rays to scatter. That's why the sky turns red and orange at sunset.