The asparagus bands may be purple so that they are clearly visible during food preparation, in the same way that caterers use blue sticking plasters for small cuts, etc. Purple and blue are generally 'no-no' natural food colour indicators, although we have become accustomed to some (blueberries, etc), and to artificial colours.
In the UK, the Royal Mail use elastic bands for their postmen to secure bundles of post, and the postman usually drops them on the ground when he's done* So the pavements tend to be littered with red elastic bands. These do deteriorate, both in sunlight and indoors. Trouble is, I don't know what they're made from... Latex, I suspect, as it's cheap and biodegradeable.
* The Royal Mail say they're not supposed to do this; they're meant to go back to the depot. Yeah, right...