My plastic tubes always discolor and get spots and I don't worry about it. FWIW, I've had one infection in seven years.
After using the tubes, you do not need to wash the tubes in iodophor - it is a santizer, not a cleanser. You use the iodophor or star san immediately before using the tubes to sanitize. After using the tubes, you wash them thoroughly and immediately. I usually do this by either rinsing hot water through them for a while, or by running some water mixed with cleanser like PBW through the tube (I usually use my pump siphon to run the cleanser through the tubing) and then rinse with hot water. With any cleanser, you must rinse thoroughly. But, if I'm cleaning immediately after using the hose, usually hot water will do the trick. Then hang the hose so that ends are pointing down and any water will drip out.