Sunlight is normally white, and as it travels through the atmosphere to an observer, some colors are scattered out of that lights by airborne particles, and that changes the final color that the viewer actually sees. The shorter wavelength components, blue and green, scatter more strongly, these colors are removed from the beam. Then at sunrise or sunset, the path through the atmosphere is longer, and that removes the blue and green, and the longer wavelength organe and red hues we see at times.
Water vapour in the atmosphere acts like numerous tiny prisms, to refract incident sunlight. Because blue light is refracted more than red, when the sun is low in the sky more of the light directed downwards less, that we see, is red, so the sky appears red.