Victor, I believe there are two roseans for the bluish color of evening shadows. One is that the sky above truly has more blue wavelengths, but also because of the reason you learned in the color class. I've heard it called "induced" color, or the complementary color that you perceive as a contrast to any strong adjacent hue.And Paolo, I've also noticed that sunrise colors are a little different. They seem distinctly pinker and less orange. The best explanation I've heard for this one is that more dust and other pollutants is mixed in the air late in the day, and the dust gives a more orangey cast. Maybe someone else has another explantation.