As an expert in astrophysics, I can tell you that the color of a star is determined by its temperature. The hottest stars are blue, while the coolest are red. The color of a star is a visual representation of the peak wavelength of the electromagnetic radiation it emits, which is related to its temperature through what's known as the black body radiation law.

The coldest stars are red dwarfs, which have temperatures ranging from approximately 2,300 to 3,800 Kelvin (K). These stars emit most of their light not in the visible spectrum but in the infrared range. However, when we talk about the visible color, the coolest main-sequence stars, like Proxima Centauri, would appear red to the human eye.

So, the answer to your question is: The color of the coldest stars is red.

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In general, a star's temperature determines its color, from red to blue-white. Spectral types are named with a letter: M, K, G, F, A, B and O. M stars are the coldest stars and O stars are the hottest.read more >>