Surface grime (this can be cleaned)
Accumulated fingerprints, greasy film, stuck-on dust, cigarette smoke, adhesive or plastic residues: over time, this layer creates a yellowish or grayish tint, especially on the blue reverse side and along the edges. It remains on the surface, and appropriate cleaning can remove it.
Cardboard yellowing (this cannot be repaired)
Cardboard and inks age under the influence of light (UV), heat, and humidity. This yellowing is within the material: edges yellowed uniformly, white turned to cream, colors faded on a card that has been left in the sun. No cleaning can restore its original color.