Well @Shamo Hybrid , remember store bought eggs for the most part are fed the cheapest available diet in order to maintain peak laying; your chickens are likely fed MUCH better thus better quality eggs - that shows in your yolks!
There are other things that darken yolks, much of those they obtain when free ranged or in large outdoor runs . Even them getting some corn will cause a darker yolk than a layer mash only fed chicken for example.