Yolk colour

All 3 of my chickens are fed the same food, organic layers pellets and brocolli or peas and meal worms mixed in with mash at night before bed but one lays a much oranger ( is that a word) yolk that the others is yolk colour down to breed or diet I was led to belive it was diet?   

The more greens they eat the yellower  the yolk i believe

All of the yolks from our hens vary in colour.  A couple are a real marigold colour, a couple are more similar to what you'd expect from supermarket eggs (without the added artifical colours to the hen's feed) and one (from my leghorn cross) is very pale indeed.  They all get the same diet and free range around a very large garden but I suspect some love their greens more than others.  I know one of my hens eats loads of greens, especially dandelion leaves (and her eggs are the deepest colour) but I rarely see the Leghorn cross eating greens.