When the sun is low on the horizon during sunrises and sunsets, the sunlight travels through more of the atmosphere. Shorter wavelength colours (blues and violets) get scattered out. This leaves more of the longer wavelength colours like yellow, orange, and red. This is why sunrises often take on such colours. When the sun is higher in the sky, smaller particles scatter much of the blue wavelength, which is why the sky appears blue. The human eye is more sensitive to blue than violet otherwise our characteristic blue sky might be violet.
