I just started using a pack of Kentona I got several years ago, and I did have trouble with the yellow blotching. The first sheet was very yellow, the second sheet was slightly yellow around the edges, and the third sheet was fine. I thought maybe it was caused by the plastic bag, since it seemed to depend on how close the sheet was to the outside of the pack (plasticizers leaching, or offgassing, or something). This was in Formulary 130.