The short answer is that you cannot tell which is laying by shell color.
The long answer. Black Stars and Red Stars are not a breed. They are not even a specific cross between breeds. Those are just marketing names that describe some Black Sex Links or Red Sex Links. There are even different types of sex links. Some are the commercial egg laying hybrids while others are crosses between two specific breeds, practically always dual purpose breeds.
The commercial egg laying hybrids have been developed by selective breeding to be fairly small in size but lay a lot of Grade A Large eggs. They have a great feed to egg conversion rate.
The dual purpose crosses are just that. They cross two hatchery breeds in a certain way that they can sex the chicks at hatch. You can use different breeds but a Black Star could be a Rhode Island Red or New Hampshire rooster over a Barred Rock or Cuckoo Marans hen. A Red Star also could be made by crossing many different breeds but some common ones are a Rhode Island Red or New Hampshire rooster over a Rhode Island White, Delaware, or White Rock hen. These are typically about the size of a dual purpose chicken and might lay a medium or large egg. They usually lay a lot of eggs since they are made from hatchery chickens.
If you know which hatchery they came from we might be able to tell you something about their ancestry but we can't tell you which lays shat shade of brown egg unless the hatchery addresses that on their web page. Or you can call the hatchery and ask them. It's possible they can tell you by shell color. I don't think very likely, but possible.
