[AlotOfReading]

It's usually easier in person where you can move your head and see the natural color variation of the rock. Plus they'll have sketched this out, looking closely at different areas, and some will be more obvious than others.

But yes, it's sometimes difficult and you have to have an eye for it.

[gus_massa]

I'm not sure what they used for painting this, but a few years ago there was a project in the university where high school students came to work in simple projects for a while.

One of the projects was to make reproductions of the crayolas used for cave painting. They mixed fat of ñandú (rhea, a local bird similar to ostrich) and iron oxide. The project was to use a spectrometer to measure how different fat, impurities and treatments changed the spectrum, probably to try to discover the original recipe and locals variations. (I don't have more details. Someone told me about this in a hallway conversation.)