On my personal world, All the grass blocks within a 16x16x16 space were several different colors. When I placed another grass block on top of the already existing ones, the ones I placed also changed color, as long as they were inside that chunk. It went back to normal when I quit out of it and rejoined. This is not the only time this has happened, it occurred once before several years ago when I was playing on a server, but I thought nothing of it at the time.