The red color comes from the mineral hematite, a type of iron oxide, basically rust. It only takes a tiny amount of iron in the rocks to give it that red color, but the iron has to have oxygen available when it is deposited to form the red color. This indicates that the sediments were deposited either on land or in shallow water.

If the sediments are deposited in deep water or stagnate water, there may not be enough oxygen available to rust the iron. in that case, the rocks would have a grey or greenish color, due to the presence of ferrous iron or undecomposed organic material. This tells us that the water level was fluctuating up and down as the rocks were deposited.