Totally protected and under surveillance. Reddish, tan, and brown dirt formed most of the colors. Primitive brushes might have been make out of animal hair. Most cave drawings are not in open entrance areas, but deep back in caves where there was no natural light. Sometimes even hidden back in passages one would have to crawl through to reach it.
Art also represents what is acceptable to a particular culture.
Future generations will be able to tell better what our art communicates. Animals have been known to make marks - elephants holding a paint brush in their trunk, monkeys, even cats by walking in paint.