Mars appears red because its surface is covered in iron oxide - literally rust. In astrology, Mars rules iron, making this connection symbolically perfect.
Mars governs action, aggression, passion, and physical energy. Iron is the metal used for weapons, tools, and building - all Martian activities. Blood contains iron and is red, linking to Mars ruling blood and vitality.
Ancient civilizations associated Mars with war gods (Ares, Nergal) partly because of its blood-red appearance in the night sky.