Actually, there's color changes in the old petroglyphs that are pretty evident just by looking at them (which is how you can tell that something's been vandalized.) In this (VERY irritating) picture, the guy has his hand all over a wall of petroglyphs (grr). To the right of his (oily, dirty, surface-contaminating (what? me??? biased????)) hand is a "triceratops."
[www.bible.ca]

Notice that the "triceratops" is composed of one, and possibly two older prehistoric images. Notice that the "horns" and the "tail" are much brighter than the body of the "triceratops" (probably a buffalo image). The panel itself is composed of what appears to be at least two culturally distinct styles plus some garbage that has been added within the past 100 years.

The fading is "varnish."

There's some beautiful examples of rock varnish on all petroglyph sites. Look at the Black Canyon site (I wish they had some of the modern graffiti to show as a contrast. The oldest petroglyphs are the color of the rock, but the next oldest are a faded pinkish-red. The youngest are yellowish to white.)
[www.petroglyphs.us]