Once when we were in the car, waiting at a stop light, a big, grimy cement truck pulled up along side. Our toddler looked at it for a moment, then exclaimed "CANDY!" Candy? Of course there wasn't any candy. But there was a round yellow light on the top of a red-and-white striped stick. To a young child, the effect was quite candy-like.
So I sat down and looked at the floor for a while, until I realized I, too, was seeing blue circles floating around on the bathroom floor, and was waiting for them to go away so I could tell her there weren't any.
The explanation was simple. There was a bright orange sticker on the floor, the kind that supermarket checkout clerks stick on detergent boxes, milk bottles, and little kids' hands. The intense orange was producing fatigue on our retinas, resulting in the blue circle effect. I explained it to her as best as I could, and since I wasn't worried, she wasn't either.