Shoppers have come up with an interesting new theory about Coles-branded eggs having a brown colour that washes off under water.

A curious shopper asked the supermarket if it was "doing something funky" with its eggs and posted a video with proof.

the video shows an egg that appeared to be brown become a pale beige colour after being lightly rubbed under a running tap.

“Hey Coles, are you guys doing something funky with your eggs these days? They look like they have been given a spray tan that rubs off when you put them in warm water or boil them,” the woman wrote.

“Before washing the residue off, all the eggs are a pretty standard brown colour. Once you wash them, they are rather different in colour compared to each other,” she explained.

Another shopper agreed, saying that they've discovered this from their own "anecdotal evidence" that some sort of "artificial colouring may be happening".

“Someone somewhere has decided that an even-toned brown egg somehow sells better. If that actually is the case, that is a new and special level of dumb,” they wrote.

Coles has since responded to the claims to Yahoo News.

“Egg colour including white specks can be influenced by a number of factors including bird age, access to minerals in the range and access to additional calcium in their diet,” the spokesperson told Yahoo News Australia.

“Our egg suppliers also sometimes use oil during the egg grading process to help seal the porous shell of the egg and preserve quality. The colouration on these eggs look to be a combination of the above effects.”