In the evening, the setting sun appears like a beautiful crimson red ball. But we all know it has not changed its color.
Sunlight contains all colors. But the atmosphere on, around and over the earth contains molecules of water, vapour, dust, air and other impurities. The sunlight which passes through these media gets scattered.
Our atmosphere scatters blue, violet and green colors much more than it does the red and yellow. Thus we can see more of red and yellow color and we have a crimson-red sky in the evening. So when Sun sets it appears red to us.