Discoloring

While soft silicone is non porous, it is not entirely impervious.  A number of things are known to be able to stain or discolor silicone, including the dyes in some pigments and long duration exposure to heat.

Silicone cubes of different materials that were either boiled or soaked in soapy water showing how discoloration appears in soft silicones.

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Examples of dyes and pigments that can stain silicone

Other causes of discoloration such as mold growth or improper storage.