Beekeepers in northeastern France have been alarmed to find their bees producing honey in unnatural shades of green and blue. The beekeepers believe the source of the problem is a biogas plant close to Ribeauville in Alsace.
It is thought the bees have been eating the sugary waste from M&Ms, small chocolates in brightly-coloured shells.
The plant operator said it regretted the situation and had put in place a procedure to stop it happening again.
"We discovered the problem at the same time [the beekeepers] did. We quickly put in place a procedure to stop it," Philippe Meinrad, a spokesman from Agrivalor, the company operating the biogas plant, was quoted by Reuters as saying.
The company, which deals with waste from a Mars chocolate factory, said it would clean out the containers, store all incoming waste in airtight containers and process it promptly, according to a company statement published in Le Monde newspaper.
A spokeswoman for the British Beekeepers' Association, Gill Maclean, said it was possible that the coloured sugar could have contaminated the honey.
Bee numbers have seen a rapid decline globally in recent years.
In the UK, a harsh winter and unseasonally heavy rain this summer has meant that bees have not been able to forage as much, and beekeepers have had to step in, offering sugar syrup, Ms Maclean said.
"Bees are clever enough to know where the best sources of sugar are, if there are no others available," she said.
As for the blue honey, the beekeepers say it is unsellable.