Yolk color is caused by what they are eating. Egg producers who brag about their dark colorful yolks feed flower petals to make the yolks darker.
I notice a big difference in my girl's eggs when they have good daily dose of grass and weeds out of the larger yard than when they are confined to the run. The taste AND look of the yolks is richer.
If possible I'd free range them more, cut back on the scratch if it is more than a handful a day - it should be a VERY small percent of their diet - only as a treat.
Personally I wouldn't feed medicated feed to any chicken past the first few weeks of chick-hood UNLESS you are in a high-cocci area, or have issues with that in your flock, or it is recomended specifically for your unusual situation.
If the medication is Amprolium there isn't a withdrawl time, per se, for eggs, so it's considered safe to eat the eggs, but I'd be reluctant to eat eggs when any medication was used, and I wonder if that may be having an effect on the egg quality. By the time chickens mature, they have developed immunity to most common strains of cocci in the environment, so it's recommended to only treat symptoms/outbreaks as they occur rather than keep them on medication continually.
