As far as I could find out, they used orange cornflour that will just wash off the next time it rains. The most amount of damage anyone could seriously bring up was that it could harm/displace the lichen on the henge.
That’s not to say that I specifically condone the action, but it’s a lot less bad than this article makes it sound. It’s the same with the soup attack on one of van Gogh’s painting, which had protective glass on it. So far all the JSO actions targeting cultural/historical things (at least the ones that made it to the big news) have been done in a way that makes them sound awful at first hearing, but intentionally did not actually damage the targeted cultural/historical thing.
I think the biases of the journalist/news outlet/etc. are somewhat exposed by which parts they focus on and which they downplay or omit entirely.