Why does antibiotics change the colour of the urine?

Most don't: Most antibotics don't do that. I am aware of only one, rifampin (also called rifampicin), which colors urine and tears red-orange. (Patients taking the drug should not use some types of contact lenses, which can be permanently stained.) There may be others -- maybe other docs will comment. If your urine is simply darker yellow than usual, it's only because you're drinking less fluids.