The red colour we see in images of Mars is the result of iron rusting. Rocks and soil on the surface of Mars contained a dust composed mostly of iron and small amounts of other elements such as chlorine and sulhpur. The rocks and soil were eroded by wind and the dust was blown across the surface by ancient volcanoes. Recent evidence points to the very fine dust also being spread across the planet by water, backed up by the presence of channels and ducts across the surface of Mars.
The iron within the dust reacted with oxygen, producing a red rust colour, while the sky appears red as storms carried the dust into the atmosphere. This dusty surface, which is between a few millimetres and two metres deep, sits above hardened lava composed mostly of basalt. The concentration of iron in this basalt is much higher than that on Earth, contributing to the red appearance of Mars.