I am not sure whether the red colour of the sandstones from that timeperiod is primarily due to elevated oxygen concentrations. Rather, after the collision of Gondwana (continent formed from what is nowadays africa ad south america) and Laurussia (Northern America and Russia formed a common continent, too) in the Hercynian orogenesis (upper Devon, lower Carbon) a new supercontinent called Pangaea was formed. Relicts from this collisions are e.g. the appalachians in US and the e.g. the Harz mountains in Germany (whence the name Hercynian). One consequence of the formation of this supercontinent is that vast regions of both Europe and Northern America, which in former times had been basically consisted of mud deposited in the shelf regions of large oceans often under oxygen deficient conditions, or plutonic intrusions like granite (also formed under reducing conditions), which formed during the collision of the continents, where now elevated above sea level and inside a huge continent with a continental climate. The errosion of these mountains which oxygen saturated water in an otherwise dry and hot climate lead to the reddish sandstone deposits from that period ("Rotliegendes" in the Permian and "Buntsandstein" in the lower Triassic).