It is normal for silicone spouts to take on a yellowish tint. According to the manufacturer, the silicone spouts already have a slight yellow tint to them, but as they are exposed to air and washing, they may turn even more yellow. They assure us that this in no way affects the performance or quality of the spout. Some spouts have more of a yellow tinge then others, so not to worry.
What you need to watch for is if the spout gets sticky or torn, then it is time to replace the spouts. Most spouts, even with daily use, will last a month or two before any signs of wear occur. Of course, this is dependent on how much your child uses the spout and if s/he is teething. Any baby that is teething is going to want to chew on the spout. We highly suggest distracting your baby at that point to avoid the spout getting torn or the valves inside the tip of the spout getting broken.
You can buy Nuby Wide Replacement Spouts by using the discount coupon code: FIRSTNUBY