The same scattering process causes both the blue daytime sky and the red sunsets; the difference is the angle and distance sunlight travels through the atmosphere.

The scattering of sunlight by air molecules is called Rayleigh scattering. During the day, when the sun is high, sunlight comes straight through and blue light scatters around, coloring the sky blue. At sunset, the sunlight hits the atmosphere at a low angle and travels a longer path. This means blue light scatters so much it doesn’t reach your eyes directly, while red light, which scatters less, dominates the view. So the colors depend on how much atmosphere the sunlight passes through.