I noticed this effect on silicone tray intended for freezing water cubes. It would seem that it should be safe for freezer, but after freezing it, cubes were partly colored by silicone part of tray. I won't use silicone for freezing anymore. Those trays for baking are not that confidence-building either. Good fast test if you should buy anything for food - if it smells of fresh car or plastic when new, don't use for food.