A simple fact to remember is that if you start with white light, then scatter away the blue, you are left with a redder color. If the sun's rays have to pass through a lot of atmosphere, it will tend to look more red. As you noticed, sunsets can look different from day to day. Sometimes they are more orange and sometimes they are more red. This is due to the amount of water vapor in the atmosphere.
Some days are dry and some are humid.
Tiny water droplets refract light, just like a prism does. The more water in the air, the more blue light is bent away. You can think of the water droplets as millions of tiny prisms. One cool way to tell whether or not there is a lot of water vapor in the air is to look at the vapor trail that airplanes leave behind. Sometimes you can see the white trail behind jets for many miles.