2r blue error of color stamp is the second issue of postage stamps for Spain that appeared in 1851. Considered as Spanish most impressive stamps, it contains error of colour that was caused by the accidental insertion of a 2 reales cliché in the printing plate of the 6 reales. This was then printed in blue giving the mistake. There are only three specimens of 2r blue error of color stamps exist today. In 1974 a fine copy was sold privately by Galeria Filatelica, Barcelona fetching price of $450,000. In 1996 a perfect mint specimen was sold for $23,200 and a good used copy fetched $15,000. Nowadays the Dos Reales of 1851 – whose error of colour is well known as Spain’s scarcest stamp – is thought as a real gem of Spanish philately.