The two photos don't have the same exposure/color balance. It's pretty obvious if you look at the plant foliage.

KNR - I think you've got close to the explanation, & I was mistaken to assume there'd been a drastic change of colour. The older picture was taken using Kodak colour film (Kodacolor, I think) - and I digitalised the left-hand picture from a print which had probably faded over the years. After reading your post, I looked at some more pictures taken at the same time as the left-hand one, and at least one of them does show some of the rocks with a slight blue colour - Click to see it

I suspect that weathering can eventually make even an ugly rock look decent, but I wonder how long it would take.