I had planned to write about the amazing things that happen when you had eggs to recipes, and why these things happened. I read several sources, investigated the possibilities, and checked the facts. What I came up with was a lot of technical jargon about proteins and foams.
Translated into everyday useable information it boils down to this: generally speaking, egg yolks act as a binder and add a creaminess to the recipe. Egg whites add fluff. This is the reason some recipes call for the eggs to be “separated” and other recipes call for whole eggs. The part(s) of the egg you want to use depends on what you are trying to achieve when cooking. For example, when making meringue only egg whites are used because all you want is fluff. For a custard, use the egg yolks for a creamy thick dish. When making cornbread, you want both, to bind the ingredients together and also add height to the bread.
The color of the eggshell makes no difference to the recipe, and the size of the individual egg may or may not be a consideration. If a recipe calls for a specific size of egg, it really means the egg volume. Organic and free-range refers to how the chicken was fed and treated, not the chemical properties of the egg, except that there may be a difference in the nutritional component.