I have a Fusion Crystal for my Nexus 6. (Blue) Unfortunately, I too have noticed some yellowing. This is a bit disappointing, considering that I had a Ringke Fusion for my Nexus 4 and it never discolored; admittedly. Admittedly, the N4 model has black colored edges TPU and then a hard clear backplate so perhaps that is why. Overall, I like the features of the Ringke Fusion for my N6, though. If it wasn't for the yellowing, I would have few complaints if any. I wonder if they'd consider making versions of the Fusion clear variant that do not yellow, or perhaps a more "Nexus 4 or 5" style version where the bumper part is non-translucent? It seems the Fusion "mint" is the closest to what I describe...does it not yellow? I'd imagine both the Crystal and Smoke Black variants, would yellow given their translucent status...even if tinted, in the case of Smoke Black?
I like the "Fusion" design whereas it provides additional protection without much bulk or visual occlusion. How does this compare to the Ringke Slim? Think the Slim in Crystal will also yellow? Also, I noticed that Spigen has a newer "Ultra Hybrid" version that seems similar to the Ringke Fusion...anyone compare the two?