Unless you have lights that are brighter than the sun, light doesn't cause corals to bleach. The heat from some lights can warm the water up and cause the corals to bleach but light in itself, won't.Too much light causes corals to bleach too.
How tall was the coral tank at the shop and how bright was their light?That’s what I was afraid of. If immediately put the coral on the rock structure, close to the light, it may bleach because of how close it is to the light.
Was their light brighter than your light?
Was their coral tank taller than yours?
Unless your light is significantly brighter than the shop's light (and that is unlikely), you don't have to worry about putting the coral on the bottom of the tank.