Your sequence of events is worrying since one hen's eggs gradually had paler yolks if the contrast between those yolks is an indication of how much it has gradually changed. A sudden change from light yellow to dark would be easy to explain, their diet changed. A gradual change to all the eggs from darker to lighter could be explained. But a gradual change in one hen's eggs but not the others when they are eating the same thing, I don't know.
They can get the pigments to color the yolks from eating different things, dark green veggies or even flower petals. Some Layer feeds contain marigold petals to color the yolks. It doesn't man the yolks are healthier or more nutritious, just that they have been dyed.
Yellow skinned chickens will use pigment from the skin to color egg yolks. As that pigment gets used up the skin can lighten. That's one way to tell which are your better egg layers if they are yellow skinned, the ones with the lighter skin are laying more eggs. But that much change in color does not come from that, that is due to what they eat or their bodies process.
I tried searching on the internet. Some diseases can cause changes to the eggs, but I did not find that yolk color is one of them. I can't point to anything specific there. Different hens digest nutrients differently, it's just the way their bodes work. If she was producing eggs with dark yolks something might have changed in the way she processes the pigments. I don't know what could cause that.
It may be nothing to worry about, but I think any unusual change is worth checking out. If you are in the USA, I suggest contacting your extension agent. If you are in another country maybe your agricultural ministry or whatever you call them can help. Try to emphasize that it is a gradual change in just one of four, that she was laying eggs with a much darker yolk and the others still are. If they try to blow you off, try to get them to put you in touch with a chicken expert. Some agents are better at that than others.
I don't know what else to tell you, I'm mystified. Good luck!
I have 4 lovely girls who are laying well, but my young brown chicken’s egg yolks have become very pale. They are all fed organic grain with access to grass twice daily and lots of greens (kale, silver beet etc) and I have observed they all eat the same. The whites appear to be a bit cloudy, compared to the others. The chooks behaviour seems fine but I am worried there is something wrong and want to be proactive in avoiding any issues. If anyone can advise what might cause this or what I should do, I’d be very grateful. Pic attached of 2 eggs from the chicken in question and one of my other chooks. Note - she previously laid normal colour yolks and the feed has no colours etc
Mine get Flock Raiser (fermented) and MW/BOSS when I close them up at night, a toss for scratching. Yolks are nice and deep yellow, once I had one that kinda of looked orange, Hubby wouldn't eat it 
So cooked it up as a treat for them, it was appreciated. 
