It's turning yellow because of oxidation. Oxidation occurs when the index card comes into contact with air. I don't think there much more I can say. :neutral:
There is a chance it may have happened when the index card came in contact with oils on your hands. The oils on your hands leave stains on paper after time.
Neither of these things happen over night, so I am stumped why your index card is oxidizing so fast. Sorry that I couldn't help more.
Good luck finding a solution!
Khackdrums

It's turning yellow because of oxidation. Oxidation occurs when the index card comes into contact with air. I don't think there much more I can say. :neutral:
There is a chance it may have happened when the index card came in contact with oils on your hands. The oils on your hands leave stains on paper after time.
Neither of these things happen over night, so I am stumped why your index card is oxidizing so fast. Sorry that I couldn't help more.
Good luck finding a solution! {up}
Khackdrums :D