This study attempts to investigate the origin of red pigments, time of reddening, and processes responsible for red colouring of the Upper Miocene age Upper Red Formation, the most important clastic unit in central Iran. Mineralogical composition and textural properties of the studied sequences show that the Upper Red Formation sequences were not red when deposited, although some parts of red pigments were derived from the source region. This study clearly shows that both detrital and diagenetic pigments contributed in red colouring of the formation but diagenetic forms were more significant.