He looked down into the bowl and saw not luscious, golden yolks staring back at him, but insipid, pale things - more primrose than sunset orange.

According to the publicity machine at Mr Walker's frozen food empire, Iceland, he went on to discover that the shade of Edwina's egg yolk has a name.

It is number 5 on the Hoffman Laroche colour scale. And the eggs stocked by Iceland scored a higher number 10.

Mr Walker and Edwina

Emma Powell of the British Egg Information Service explained that because supermarkets are "consumer driven" to stock eggs with rich orangy-yellow yolks, farmers are obliged to give pigmented feed to their chickens.

And that feed is picked from a colour chart similar to a DIY paint colour chart.

If anyone ever wanted to, it would be possible to produce red-yolked eggs, with a feed labelled at No 15 on the scale.

Mr Walker's response to the same information was to withdraw artificially coloured eggs from Iceland's shelves. Customers, he said, just didn't want adulterated food.

The supermarket chain has announced that it has removed all artificial colours from its own name brands. Spokeswoman Jeanette Riley said that artificial colours have been replaced by natural ones.

'Naturally' flavoured chocolate carrots

Blackcurrent sweets, for example, are no longer coloured artificially, but with a natural blackcurrant extract.

Similarly, their Indian ready meals are coloured with deodorised paprika and turmeric, and sausages with carmine instead of the colour Red 2g.

Its Wacky Veg range - including chocolate flavoured carrots and pizza flavoured sweetcorn - never contained artificial additives, added Ms Riley.

But many other shop shelves are laden with fresh foods, including meat, fish and cheese, which have been coloured to enhance their desirability.

There are three different agents used to tinker with the colour of egg yolk - canthaxanthin (E161g), citranaxanthin (E161i) and beta apo81 carotenal (E160).

Canthaxanthin was feared to cause retinal damage in some people taking self-tanning pills, and was banned for that purpose. But because it is consumed in much smaller quantities in eggs, it is still legal to feed pellets containing the artificial colour to chickens, so that their yolks will look more appealing.

Farmed salmon can be fed pellets to make them more pink

However, Ms Powell said that to "avoid confusion", the Lion quality mark guidelines - set up by the Egg Information Service for its members - do not permit the use of E161g.

But the remaining 30% of British egg producers are at liberty to use the colourant.

Salmon farmers can also feed the colour to their fish to make their flesh pink, says Dr Tim Lobstein, co-director of the Food Commission. "Presumably, it would otherwise be a dull grey," he said.

And according to organic farmers, animals farmed for mass produced meat can also be fed pigment-enhancing pellets that make their meat more pink or red.

Iceland says that it does not stock meat from animals fed on coloured pellets - and says it is working on a programme to phase out the practice by its salmon suppliers.

Dr Lobstein points out that artificial colours have been used to make food look more attractive for about 150 years now.

Organic chocolate - no artificial colours or flavours

He said that in the late 19th Century agents including red lead were used to change the colour of tea. The practice was stopped after the health implications were realised, and adding artificial colouring to food was banned.

However, he said, more and more artificial colours have got back into food production - dyes and pigments made from coal tar and petroleum derivatives have also appeared in the manufacture of foods. Tartrazine, for example, is obtained from coal tar.

Today it can be extremely difficult to find items on a supermarket shelf without them in.

But while your average pot of pasta sauce has to have its contents - including additives - clearly marked on its label, any colour which is not strictly "an ingredient" does not have to be owned up to.

So meat, fish and poultry products which are bright pink, red or yellow because the animal which produced them ate pigmented feed, does not have to have E161g - for example - stamped on its wrapper.

"The thing with eggs," said Ms Powell, "Is that people assume that if they are darker, they are fresher, and that isn't really the case.

"But supermarkets are driven by what consumers want, and since the early 70s, they have wanted deep golden egg yolks."

What would a modern larder look like without artificial colours?

Coincidentally, the early 70s also saw consumers begin to demand eggs with brown shells, instead of the then normal white ones, because like the new brown bread and pasta, they were perceived to be healthier.

Dr Lobstein says that consumers are right to be worried about artificial colours in their food. He says that they could have health implications, and more immediately, serve to divert consumers from more nutritious food.

"Highly processed food has to have them in so that they look palatable. Then this processed food looks more attractive, which encourages people to eat more of it.

He added: "Let's not get anxious about this, let's get angry. These big companies are putting these things into our food and passing it off as healthy and nutritious."