Not all light travels at the same speed. The rate of speed cited in other answers here is the speed of light in an absolute vacuum. In the presence of air, or dust, it slows down or is blocked altogether.
Refraction happens when a clear substance slows down light to the point where it separates into an array of different frequencies, which you see as a rainbow.
Red is the slowest frequency in the light spectrum, which is why astronomers rely on a "red shift" to determine if a planet is or is not receding. The redder the shift, the further away it has traveled.
The red shift in light is akin to the dopplar effect in sound.