My year-old Spigen has yellowed edges. Looks horrible against my silver phone. I have the, "Spigen Ultra Hybrid Nexus 6P Case with Air Cushion Technology and Hybrid Drop Protection for Nexus 6P 2015 - Crystal Clear". I love the design, it's nice and slim, removes easily for cleaning, has never scratched the phone.
I've purchased clear cases for my last 4 phones. All the cases, from different manufacturers, have yellowed after a year. The 'good' part is my first case was 3x the price of the one above and it's now, a year later, even less. So, I'm buying it again.