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Eggshell colourThe colour of eggshells is the result of pigments being deposited during egg formation within the oviduct. The type of pigment depends upon the breed and is genetically determined.In 1933, Professor Punnett demonstrated that the blue egg factor is a dominant gene (genetic symbol O).All eggs are initially white, and shell colour is the result of the pigments called porphyrins being deposited while the eggs are in the process of formation. In the case of the Rhode Island Red, the brown pigment protoporphyrin, derived from haemoglobin in the blood, is what gives the shell its light brown colour.The Araucana produces a pigment called oocyanin, which is a product of bile formation, and results in blue or bluish-green eggs. Interestingly, the colour goes right through the shell, making the eggs difficult to candle during incubation. This factor is also an indication of the relative purity of the stock in relation to original Araucanas. The original shell colour of Araucana eggs is blue, but a variety of colours have been produced by crossing Araucanas with other breeds, as followsAraucana eggshell coloursBlue X Blue = BlueBlue X White (eg, Leghorn) = BlueBlue X Brown (eg, RIR) = GreenBlue X Dark Brown (eg, Maran) = Olive BrownThis is a simplification, of course, and there is a considerable range of blue - green - olive hues, depending on the genetic make-up of the breeds involved, bearing in mind that many so-called Araucanas are themselves crosses.Children are often fascinated to learn that you can rub off the eggshell colour if you get to the egg as it is laid and while it is still damp. Once dried, no amount of rubbing or washing will remove it. Some producers of brown speckled eggs prefer not to use wood shavings as a nest box liner because they smear the markings before they have a chance to set.Colour preferencesMost people in Britain prefer brown-shelled eggs – at least they did until Delia Smith mentioned white eggs on one of her TV cookery programmes. In the USA and Spain white eggs are preferred. There is no relationship between egg quality and shell colour. Nutritionally they are the same, but you’d be surprised how many people still think that brown eggs come from free-range hens while white ones come from batteries!Selling cartons of different coloured eggs is popular with small producers. It’s possible for example to have a 6-pack carton containing a dark brown speckled egg (Speckledy or Maran), a pure white (White Star or White Leghorn), a pinkish-brown (Rhode Island Red), a creamy white (Ancona or Vorwerk), a mid-brown (Black Rock or Barnevelder), and a bluish-green (Araucana or Cream Legbar). The latter are particularly popular at Halloween.Pale eggsAlthough shell colour is mainly determined by genetics, the effect of strong sun and high temperatures on the hens can produce a fading effect on the shells. Why too much sun affects the surface pigmentation in this way is unknown, but it can be a problem for those who sell such eggs.The Maran, Welsummer and Speckledy breeds lay dark brown, speckled eggs. One way of dealing with the problem is to confine the chickens for a few days, but this may be difficult for those who are selling eggs described as free-range, where the chickens are supposed to have unrestricted access to the outside during the day.Pale shell colour is much better known in hot countries than it is in our northern climes, for obvious reasons. It has really only emerged as a topic for discussion in Britain since free-range management made a return to the commercial sector. Traditional knowledge, as for example in my parents’ generation, has always recognised it, hence their emphasis on the importance of extra shade and cold water to drink in the summer. Their knowledge was based on experience rather than on experimental research, but the subject has attracted the attention of scientists in recent years.Research in Australia has shown that providing water at a temperature of 5OC in very hot weather enabled hens to produce eggs that maintained their dark shell colour. The shells also had a better weight and shell breaking strength. (Ref: Shell Quality and Cooling Drinking Water. Tangkere, Bhandra & Dingle. University of Queensland. 2001).There are also other reasons why shell colour becomes paler. Stress, for example, can affect the colour intensity. A sudden disturbance to the normal routine may result in a hen retaining the egg within the shell gland area of the oviduct for a longer than normal period.During this time a very thin layer of extra calcium is deposited on the egg, producing a greyish, bleached out look. By the same token, an egg that is laid prematurely may not have had enough pigment deposited. Stress can be caused by many different factors, including sudden changes to routine, moving to another environment, change to the diet and shocks such as loud noises, bullying within the flock or the presence of predators.Diet is important in producing quality eggs. There needs to be a well-balanced ration that provides the whole range of nutrients, vitamins and minerals. For free-range layers this is the provision of a proprietary compound feed used in association with grass paddocks.We already know that grass and other plants such as clovers and lucerne enhance the yellow colour of the yolk. What is also likely (although not much research has been carried out on this aspect) is that they also contribute to the vibrancy of shell colour. A good quality organic or free-range feed is essential, while the grass should be clean, short and with plenty of new growth tillers (tips).The presence of the coccidiostat Nicarbazin in the feed can produce paler shells. Again, make sure that only good quality organic or free-range feeds without this additive, and with the ingredients clearly listed on the label are used.Viral infections can lead to loss of shell colour, as well as egg deformities and a reduction in egg numbers. They include Infectious bronchitis, Newcastle disease and Egg drop syndrome.Older hens are more likely to produce paler eggshells than younger birds. Replacements will need to be made available on a regular basis if egg quality is to be maintained.The presence of parasitic red mites that emerge at night to feed on the perching hens can have a debilitating effect on the birds, leading to anaemia and the loss of egg colour. Check the house and birds regularly. There are several proprietary products available for dealing with red mites.Finally, the presence of parasitic worms can debilitate the chickens to the extent that their nutritional intake is affected. This, in turn, leads to the production of pale eggs. A wormer such as Flubenvet added to the feed is effective in getting rid of internal parasites. After treatment the birds should go out onto clean pasture that has not been used by poultry in the same season.
I moved each of the cockerels in turn into a pen with Warrens. The cockerels carry the genes for coloured eggs and hardiness, the Warrens were good egg layers. The combination, if I were lucky, would produce hardy hens, regularly laying large pretty coloured eggs. My first generation hens from the Araucana crosses laid eggs which were light olive green in colour. The hens were small or medium framed, but they laid large eggs, as big as the Warrens. Several generations later the hens lay blue or green eggs, are small to medium size, and hardy. Some of the hens are quite tiny but still lay large eggs.The hens have very mixed parentage and come in colours from white through to black and mixtures in between. It doesn't matter about keeping the hens as separate colours, since the important selection factor is the egg colour, so I don't need to know which hen has laid which egg. The result is constantly surprising (isn't that half the fun?), producing some truly beautiful hens, although being mongrels they will never win a prize at a show.
Yep - pretty brown speckled hens and fluffy snowy white hens make pretty white eggs  well, not quite but it's easier to remember than the ever clever (george, was it??) another_someone 
.....What the real truth is about the colour of eggs is that when an egg is laid the egg fairies come and paint it !!..it's as simple as that !!..sheesh !!!....obvious now isn't it ? 
Does anyone have any idea why egg white should be a solvent for the shell's colouring.