PS, Good question. I did a Google search and found the following.
S.Maria_Maggiore
However, I must provide my own opinion. I have visited some of the tombs in Egypt, and the colors of the paintings are as fresh today as if painted recently, but in actuality they are thousands of years old. I believe they have preserved their vital colors, because they have been buried in darkness for so long. The same techniques used at temples with similar paintings have been bleached from the elements including sun light, because they are exposed to the open air. c.i.