Question

why is Mars called the red planet

Answer 1

because it appear in the sky as on orange red star. the colour caused the ancient Greeks and Roman to name it after their god of wars.

Answer 2

photos from spacecraft show that it’s a rusty red color. The hue comes from the fact that the surface is *actually* rusty, as in, it’s rich in iron oxide.
So that's why mars is known as red planet