Now that I know the brand, Donmoor, I'm finding more about the kids' '70s shirts online. Here's a link to another forum if this is allowed (I'm new at this).
They get into the whole Kirk-avocado thing, but also talk re. the true-green shirt Donmoor made. Then they extrapolate who might have worn such a green shirt in Starfleet. There's a picture of the green shirt on one of the pages somewhere. Have fun.