I was quite surprised on a number of occasions that a TWOC was carried out when the bag was quite red. I think it goes to show it often looks worse than it is.
At our hospital once the catheter is removed they require two samples of a reasonable amount, my last TWOC I only managed small amounts so had to do three samples.
Stay relaxed and you should be fine. Drinking the water too quickly can allegedly slow process of passing urine. Having had a number of TWOCs I slowly drink water in the couple of hours prior to the TWOC.
Make sure the catheter is not yanked out, just smile at the nurse and say nice and gently no yanking,if nothing else it focuses the nurses mind.