Eggland's say on their website that they add lutein supplements to their chicken feed.

Lutein is a yellow colored carotenoid and will improve the yolk color. Their feed supplements may also include other carotenoids.

Depending on how far north you live there may not be much grass growing at this time of year for outdoor hens to eat.

The colour of egg yolks is easy to manipulate with carotenoid supplements in the chickens feed. There are guides available showing you how to add different ratios of colored caroteniods to chicken feed to get whatever colour you want in your egg yolks.

http://www.christa.bg/files/Catalogue/105.pdf

They even provide a color fan to help you achieve a consistent color.

I have a flock of ten laying hens, and the color of the yolks definitely changes throughout the year based on what is available for foraging. My older hens molt and rarely lay all winter, but my newer hens who hatched this past spring have slowed down a bit, but we still are getting a few eggs a day without artificially introducing a timed light (supplementing to create twelve hours of light will keep them laying year round). So the eggs we collect now have lighter yolks than our spring eggs.

Diet is important for egg color and quality. As Matthew points out, feeding certain phytochemicals will positively affect egg yolk color.

You also need to realise that chickens (and most all livestock in general!) laying in fall and winter aren't going to be getting much their nutrition from pasture, and will require supplemental feed (i.e. a grain ration).

According to Barb, the poultry lady at the farm market, early spring is the best time for bugs and grass, and therefore the best eggs. I'd guess that your theory is correct.

If the hens are fed greens the yolks get brighter yellow. In the winter they could get supplemented with veggie trimmings and that would help. Guess that is harder to do on a large scale.

I am old enough to remember when there was a huge difference in color of the yolks depending on the time of the year. In summer, the yolks were very bright, but in January, a boiled egg would yield a green yolk.

I think that generally, but not always, pastured eggs have a darker yolk color. More importantly though, the yolk should be firm and stand up, when you crack the egg into a pan for example, the yolk should be almost a half-sphere rather than a flattened blob. The yolk should hold together well and not be punctured easily, and if you separate them, the white and yolk should separate readily. Also, generally, pastured eggs are smaller with a lower ratio of white to yolk than the huge supermarket eggs.

The yolk should taste very creamy and eggy. This is subjective, since any yolk tastes creamy, but you should be able to taste the difference, I find pastured eggs are to supermarket eggs as cream is to half and half.

I agree with other posters that they can probably fake the yolk color somehow, but they can't fake these other things. Also, sadly, not all pastured eggs are really better. I bought pastured eggs from a local farm for a while, and later learned that the chickens were fed standard chicken feed with a little bit of extra soy added, and "no animal products or by-products". I am not sure if that means no bugs, or what, but I'd actually prefer that the chickens ate an omnivorous diet, since that is their natural diet, and I think it produces better eggs.

Rhode Island Reds, Black Stars, Isa Browns, and Red Stars all lay daily without the addition of artificial light - at least mine do in central Indiana. During the winter there is nothing for the chickens to eat if they are pastured, so I feed them exclusively mixed hay and alfalfa hay from my own pasture.

If someone is selling pastured eggs in the winter, they're probably doing the same as me.

FYI, the reason for the different breeds is since they are free range, cage free, pastured, I can tell at a glance how old each hen is. My hens stay with me 2 years and I hatch out new stock every 6 months in the order listed above.