No, it definitely doesn't yellow or discolor over time. Okna uses a very high grade virgin uPVC extrusion with thick outer walls..you are not going to get the discoloration and warping that's common in lower quality products.
They don't use cardinal, they use guardian.

This post was edited by mmarse1 on Fri, Feb 22, 13 at 22:02

+1. The vinyl thickness and composition of the Okna product is second to none. They use Guardian glass which along with Cardinal is one of the top choices available.