According to wikipedia, the colour of soli is determined by proteins present in the soil!
Soil color is influenced by the amount of proteins present in the soil. Yellow or red soil indicates the presence of iron oxides. Dark brown or black color in soil indicates that the soil has a high organic matter content. Wet soil will appear darker than dry soil.However, the presence of water also affects soil color by affecting the oxidation rate. Soil that has a high water content will have less air in the soil, specifically less oxygen. In well drained (and therefore oxygen rich soils) red and brown colors caused by oxidation are more common, as opposed to in wet (low oxygen) soils where the soil usually appears grey.
The presence of specific minerals can also affect soil color. Manganese oxide causes a black color, glauconite makes the soil green, and calcite can make soil in arid regions appear white.
It sounds a bit strange to me. Pigments proteins do impart colour to the substances they are in (e.g. Haemoglobin in blood) but I wonder if there are any such proteins that could impart the variable colours to soil. Besides the following sentences in the mentioned paragraph do not mention of any protein.
It is possible that the word protein is there by mistake?
P.S. The book from where the sentence is quoted is unfortunately not available online.