Why is sunset or sunrise sometimes red and orange and colorful?

My science teacher says pollution, dust, and dirt make it more vivid. But, my step-dad says that my teacher is wrong because of the saying, "red sky at night, sailors delight, red sky night in morning, sailors warning" which dates back before pollution was a problem. Also, i remember in school the teacher said that an effect of volcano eruptions is vivid sunsets and it's from the ash in the sky. I think my teacher is right, but I'm not sure.