As egglayers hatch out with little to no color, the first livebearer species that comes to mind that could be solid orange/red from birth is a red Swordtail.

But that begs the question: Where was the mother? Unless, some the the plants had been in a tank with a female who had given birth and when the plants were moved, they took a baby with them.

Trying to play "Guess the Species" under the conditions you described could indeed be difficult. If you are really curious, perhaps the shop would agree to catch it from the plant tank and place it into a small clear container for close examination.

Top two choices are either a baby platy or swordtail both of which are common in fish stores. Many times an employee will catch out babies from the main tank and put them in a plant tank where they are safe.