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The color of the yolk is due to substances called carotenoids. The nutritional value of the egg is not affected by the yolk color..
T…he most important sources of carotenoids in poultry feed are corn, corn gluten, alfalfa (lucerne) and grass meals; all these sources contain the pigmenting carotenoids lutein and zeaxanthin, which, together with other oxygen-containing carotenoids, are known by the collective name of xanthophylls. Artificial yellow and red carotenoids are often added to feed in order to maintain a consistent dark yellow as consumers often falsely believe that egg yolk color determines quality. ( Full Answer )