A couple of things. Black Star is a marketing name that just means it is a sex link. It could be a hybrid egg-laying commercial type, those are not known for long life, but if they do live they can be pretty productive, even later in life. The other type of black sex link is made from crossing two regular breeds, maybe a Rhode Island Red rooster and a Barred Rock hen but other breeds can be used. These are no different than a regular dual purpose hen. If you know which hatchery she came from you may be able to go online and see which she is.
The basic egg shell color will not change. That is set genetically. They are either blue or white, if brown is added then green or brown. The genetics that control the shade of brown can get messy. There are a lot of different genes that can be involved, whichever genes the hen has sets the basic shade of brown, all the way from a pink tint to a deep chocolate. That basic shade is set by genetics.
But what can change is shade of green or brown. The way the process works the brown is typically added after the shell has been formed during the last half hour or so that the egg is still in the shell gland. If the hen lays the egg a little early then all the brown might not all get put on. That can give different shades of brown from one day to the next. The hen only makes so much brown in a day so a delay in laying may make more brown available. If the hen lays a double yolker that brown can get spread kind of thin, a small egg it may be darker than normal.
Another thing that happens is hat over a hen's egg laying cycle the eggs get lighter. When she starts laying after a molt the eggs are typically as dark as she will lay. Over the weeks and months the eggs can gradually get lighter. As hens age some of this stuff can change too. A lot of this won't apply to your eggs. If both are laying it would be polite of them to both lay in the same day so you would know.
I haven't tried it myself but I've read you can put some food coloring inside a hen's vent in the morning before she lays and mark he eggs that way. I've also heard about the lipstick but no way am I going to try to explain to my wife why I'm putting lipstick on a hens vent.
Another way to check if a hen is laying is to look at her vent. If it looks soft and moist she should be laying. You can't tell how often, just if. If the vent is hard and tight she is not laying. The difference is pretty obvious when you see it.
It's hard for me to look at that photo and tell but if you are consistently getting two different shades of brown it's probably two different hens.
