The colour of an eggs shell is genetically determined by the breed of chicken that lay the eggs. It has no direct bearing on the shell and internal quality of the egg. Technically it is determined by the amount of porphyrines (normal degradation product of red blood cells) that is embedded in the shell during formation. Why do some people prefer a certain shell colour? It boils down to what you are used to use. A white shelled egg looks very “different” to a person used to brown shelled eggs and vice versa. Historically, white shelled eggs where easier to candle(checked over a light source-originally a candle) to establish whether meat/bloodspots might be present in the egg, without destructing the egg. That lead to the belief that white shelled eggs are less prone to have pieces of blood/meat included in them. Nowadays production and quality control methods have evolved to the point where no difference in occurrence are found.