"Too Warm" sticker for cold food

During packaging, a temperature sensitive sticker is added to the product; when the food reaches a temperature outside the "safe zone," it changes color (e.g., from white [safe] to red [danger]).

Different color codes could be used for frozen (-32°F, 0°C) or refrigerated (45°F, 7.2°C); e.g., blue/red for frozen, white/red for refrigerated).

The purpose would be to alert the grocer if the food has gotten too warm during transport, or the consumer if it has been too warm in the store.

This is prompted by an experience I had some years ago, when I discovered that the owner of a convenient store was turning off his refrigerated storage at night, to conserve electricity...