I have noticed that every egg Nigella uses has the deepest golden colored yolk and I have bought every brand of brown egg we have locally and the yolk never looks like that! Is there a reason that the yolk is so golden ie. type of chicken, age of chicken..?
Nigella likes to use eggs that have a very yellow yolk - the yellow comes from catenoids which also means the yellow yolks are a good source vitamin A. The colour of the egg shell doesn't have any influence on the goodness or quality of the yolk and white, the shell colour is influenced by the breed of hen and in the UK brown eggs are more popular with consumers so are sold by supermarkets. The colour of the yolk is mostly determined by the diet and lifestyle of the hen - the more varied and nutritious the diet the yellower the yolk will be. Italian hens are often raised very naturally on free-range farms so are able to roam and have a varied diet, his gives them the very yellow yolk.
In the UK and the US you are likely to get the yellowest yolks from free-range hens and it may be worth trying a local farmer's market for eggs. If you have a good local Italian deli then they may also stock eggs imported from Italy. In London we would suggest Michanicou Brothers in Holland Park as a source for Italian eggs.