My understanding is that its the greens that make the dark-colored yolks. Can you let them out to eat some grass? Ours are only out about an hour a day, plus greens I feed them, and their yolks are amazingly DARK orange-gold.
Of course, don't do this if you spray you lawn with chemicals!
If you can't let them out, try giving them greens. We feed any scrap or wilted greens from our own kitchen. Plus the other day I went to a natural-foods store and asked if they had any old/wilted greens I could have for the chickens, and they gave me a whole big box packed with bunches of organic kale, chard, and spinach. Not rotten or moldy (bad for chickens) just a little too past-prime to sell. I have been giving them one bunch per day and it's more than making up for the fact that they're eating less grass now that its winter. Yolks are still dark.
You might try asking the produce guy at your local grocery. Non-chain stores are the better bet.
The greens also boost the levels of healthy Omega-3s in the eggs!
Stacey