Anyone have this happen or have any ideas how this happens?
The base color does not change. A hen will lay either a base white or a base blue egg. The egg develops in the shell gland a base color, then during the last half hour the brown that turns it brown or green is added on the outside. How much brown is added determines the shade of brown or green. In your case both are laying base blue. If you open the dark green and light blue/green eggs and remove the membrane from inside you should pretty much see the same shade of blue.
Why would one hen apparently lay a light green one day and a dark green the next? Maybe she's not. It's possible a hen can lay two eggs in one day. I don't know how often this has happened to you but doubt this is it. If one was occasionally laying two a day you'd probably get three colored eggs in a day at least once. If a hen accidentally releases two yolks in a day she can lay two separate eggs the next day. That's how you get double yolked eggs. If she accidentally releases two yolks at the same time double yolked, spread out release gives you two separate eggs. Often, if she lays two eggs in one day there are marks on the egg where they were touching in the shell gland.
A hen often makes enough shell material or brown pigment for one egg. When she lays two in a day the second one is often thin-shelled or it can be a lot lighter. Have you noticed a difference in the shell thickness when you get to light ones in a day? It could be that your lighter layer is laying two when the dark one skips or the light one skips when the dark one lays two with the second lighter. As I said, I don't think this is it but it's worth mentioning.
I think this is more likely. The way the brown color goes on an egg, if the hen pops it out early (before she has time to put the brown layer on) the egg can be a lot lighter than normal. Some kind of stress might cause her to do this. It could be strange sounds, maybe a suspected predator scare, a change in lights if you provide artificial lighting. It sounds strange that yours would do it twice in a week.
Any hen can do either of these at any time, they are basically an oops anD I think we are all entitled to an occasional oops without people being too critical. If it becomes regular then there is something going on. Pullets just coming into lay are more prone to both of these, it sometimes takes them a bit to get all the bugs worked out of their internal egg making factory.
There are diseases that can cause changes in eggs but usually you get a lot weirder changes and it is more than one hen. I hate to mention this because it might scare you unnecessarily. As long as they are acting normally it is extremely unlikely they are sick. But if you notice changes it's worth paying attention.
Something else that is not affecting you since you are quickly getting this change. The longer the hen is into her egg-laying cycle the lighter brown or green the eggs typically get. When a pullet first starts laying or a hen lays her first eggs after a molt is the darkest those eggs will be. As the weeks and months pass the eggs tend to get lighter. I've had hens lay an almost white or blue egg just before the molt when early on they lay a brown or green egg. I don't know why that happens. The raw material they use to make the pigment is recycled dead red blood cells. Red blood cells are dying and being replaced all the time so there is no shortage of that raw material but my shells typically get lighter as time passes.
I don't know exactly what is going on with your eggs. To me there are normal natural explanations. As long as they are not acting sick I would not worry.
a Easter egger girl I hatched went from laying a watercolor blue to an olive green for the first 12 moths after stating laying before settling on green
