There were various techniques of wall painting, but it was very likely the period and place you are referring to is fresco. The encaustic painting, using wax, was usually painted on wooden panels and was a 'higher' art than the wall painting. Sometimes the fresco was coated with beeswax or olive oil, but eventually this died out. Likewise, most statues were annually touched up with lifelike paints, realism was the goal. Much of the ancient world would look like an amusement park to our eyes if we were magically transported to that time - gaudy colors and ostentatious architecture. The mistake in plot is the 'whitewashing to protect against weather'. The frescoes were sometimes painted over because there is always a period of history when the past looks old fashioned, they wanted to brighten up the joint. Even to our eyes today, many of the saints of Byzantine period look downright spooky - they seem distorted and unreal. The fresco, on its own, is as durable as whitewash and would certainly weather well open to the Med climate. There are some old pigments which were not especially light fast (the legendary lapis lazuli for example) but the enemy here is the sun UV light. Whitewash dries to a material as hard as limestone or marble, but it is weak to acids, the acid rains which have plagued Europe have aged the calcium carbonate materials the equivalent of many hundreds of years in less than 100 years. This same acid can be used to slowly dissolve the whitewash layer (by the way, orange juice is an acid). If the caretakers had hoped to preserve the fresco, they would have used a protective layer like a sugar water or something like that.

Kremer-pigmente.com is a source of European preservation materials. True, the Getty resource doesn't tell you exactly how to order an abstract, but this what academics do all the time - if you could only get a research grant to travel where the libaries are!