Here's why:

* Temperature and Color: A star's color is directly related to its surface temperature. Hotter stars emit more blue light, while cooler stars emit more red light.

* Spectral Classes: Astronomers classify stars based on their temperature and spectral lines. The coolest stars belong to the spectral class M, followed by K and G. These stars are often referred to as "red dwarfs."

* Examples: Our Sun (a G-type star) appears yellow, while cooler red dwarfs like Proxima Centauri appear reddish.

So, while the term "cool" is relative (even red dwarfs are still incredibly hot!), the coolest stars are indeed the reddest!