The bright orange-red colour of the rock surfaces comes from the oxidation (or rusting) of the iron in the arkose. When fresh (not exposed to the air), the arkose has a grey colour. Uluṟu appears to change colour at different times of the day and year. It glows a deep red colour during sunrise and sunset. The red light that comes across the sky at these times reflects off the rock, sand and clouds to make a bright glowing effect.