To me, this sounds more like an altar rather than a ritual. Though perhaps it could be the last of a ritual that someone didn't bother to clean up. I would recommend you leave the candles be though; incase it is for someone to practice in the privacy of an abandoned center, or if it is their sacred space, etc.
As Fredek said, coloured candles could mean almost anything depending on the creator. Everyone has their own interpretation as to what each colour symbolizes. However, typically, green indicates balance of emotions, luck, abundance, prosperity, growth and healing. It also seems to help to establish calmness.
Red tends to give people the idea of power, vitality, strength, passion and love matters- as well as several other strong emotions. Perhaps, since you mentioned it previously was an alcohol rehab center, that this could include addiction?
As also mentioned above, many addicts took this time in rehabilitation to pray, or look to spiritual work in order to aid them in this tough time. The candles and paint could possibly be the remains of such.
I would take a look and see if the paint is clean, fresh and new, there's little dust, etc. This suggests recent activity within that area, as if someone is still using it. If so, and if you still remain curious, perhaps you should visit now and again to monitor any changes or sign of happenstance.