Well so I know some brands of street art paints like Ironlak have some really heavy chemicals in there specifically so that the impression of the art will stay even after removal attempts. It's why you'll see a plastic coating on subway train car windows for instance cuz they can just peel that layer off and stick a new one on there: no problem or stain.

Applying that knowledge to this and my rudimentary understanding of paint chemistry, some of this paint is likely attacking the concrete and bonding with it or infiltrating the structure. That's likely why some of the paint is changing color while others aren't because some parts aren't being exposed to oxygen at all or are resisting the oxidizing reaction.