Yellow colours: Often pure sulphur (The Krysuvik Area).
Red and brown colours: Iron and sulphur (Fe, S), which assimilates with the oxygen of the atmosphere. The sulphur creates yellow colours and the iron red and brown hematite.
Grey colours: are created when sulphuric acids dissolve the basaltic rock. H2S assimilates the oxygen of the ground water and creates H2SO4 and clay minerals are created.
Light colours: are mainly created by Silica (SiO2), Calcium (CaCO3), and sometimes also Gypsum (CaSO4, H2O).
Photo Credit: Must See in Iceland