If the frog was just a darker color it could just be an indication of stress from being in the pet shop. If it was a fairly new animal it's definitely possible that it was just scared and so it changed it's color to blend in, or it was just a reaction to it's emotions. Unless there were any blemishes or discolorations on it's skin I'd say it was healthy, though a picture could certainly help us to decide. Were one or both of it's eyes open? Like it might be paranoid? My WTF slept like that for the first few weeks after I got him, like he was watching out for predators, and he was always a darker color until he settled in.

Also, if you do purchase a new frog, especially from a chain pet store, it would be best to keep it separated from Orchid for at least 30 days for quarantine to make sure you didn't buy the frog with any illnesses. If it does end up being sick, a quarantine will make sure Orchid doesn't catch anything it may have, like parasites or a bacterial infection. I quarantine all my frogs for 30 days to be sure they're not sick, and if they start to show symptoms you'll only have to take one of them to the vet as opposed to both to make sure the illness isn't spreading. A temporary home for a new frog is always an option, just as long as it's big enough and feels safe enough for the frog to be in for a month.

Hope that helped, just some thoughts.