The water in the patient's cup, which is in the dental unit, has the color of antiseptic tablets, which are dissolved in it. Various colors - green, yellow, orange, purple, blue .. The liquid is intended for rinsing the mouth before / during / after treatment. You can't drink it.
There is usually distilled water in bottles next to the dental chair, which may vary in color. Or it could be a solution of potassium permanganate, if you are talking about a purple liquid in a rinse glass.