The advice at the Garden Center was good - the darker leaved wax begonias do better in full sun than the green leaved ones.

I'd recommend no more than 2 begonias for your railing pot. I think it would look especially pretty to add a trailing plant too. A simple variegated vinca vine would look nice. Take a look around the Garden Center for what may be available.

I've grown wax begonia in a shallow dish garden container and was amazed at how drought tolerant it was; mine survived and thrived so I think they should do well in your container too.

Whatever you plant will be enhanced by the lovely background plants already blooming in your picture!

I'd remove the container from the railing. IMO it detracts from the
lovely tiered gardens it blocks. Have you considered adding some annuals among the plants that are there instead? You could use that container along the ground in front of where it is now. Or maybe a large pot next to the steps.

The dark leaved begonias in the "Cocktail" series would definitely do well there. They are easy and heat tolerant. I think you could easily grow 3 or 4 in your pot. They will fill in more quickly and give a nice full effect. They are also easy to start from cuttings. I bought a 3 pack that had gotten a bit leggy. I pinched the tops out of them and stuck the tops into the soil in the pot. With daily watering, they rooted and started growing in just a matter of days.

Dragon wing begonias would also do well. They have more of a cascading effect, the leaves are larger, and the blooms are more numerous and of a different form than the Cocktails. They come in basically the same color range as Cocktail.

Your garden is lovely. What is the blue bloomer? Does it bloom all season long?

I like the container where it is....sort of like the end cap of the railing. It's a specialized container, designed to fit snugly over a railing.

What are those beautiful blue plants growing in your beds behind the rail?

Something yellow or orange to bring out the blue. Pink ...maybe?