There are a number of possible explanations for the rare paint contaminant, if that is what the iron oxide is. Quite often an inscription concerning the dead was written in red paint above the niche (or kokh) where the body was to be laid. Blue paint was used over earthen graves of the poor to warn people that a dead body was there and the area unclean. Yellow was often used on ossuaries. In addition, the tomb caves were often decorated with frescoes of various colors. Any or all of these could have found their way onto the shroud in microscopic amounts. In addition, medieval copies of the shroud were laid on top of the shroud to produce “touch relics”; these could also have been sources of some types of paint contamination.