Weathering, plants, transport or geologic layers can all explain such, however I suspect this is some sort of Lichen discolouring the stone, as you can see it peeling off 4 stones above the pretty lady in the middle. Check out the rocks in your quarry example again. One on the top is a distinct red, a different colour from the others.
North of me, in WOH country, there is a beach called Rainbow Beach. The rocks erode away and give off coloured sands, creating quite an interesting effect in some places.
OOPS, sorry about that, an accident I swear, it is Rainbow Beach though, so I will leave it for perspective - hang on
Lovely isn't it! Quite common too. The below formation is in Hong Kong.
This is not coloured, but a geological formation that has always impressed me immensely. I hope to see it in person some day. Nature is indeed an artist.
Edited by psyche101, 22 January 2013 - 11:39 PM.