Are you looking at annuals or perennials?

I was thinking perennials.

Joey, try Camillias, they love shades and have wide variety of colors. Also try Arabian Jasmine var "Grand Duke" . Good luck

The Camilias look beautiful! I have never seen them before.

Joey

Joey, I saw beautiful Camillias trees or bush form at Home Depot the other day. They come in red, pink, or white. We use to have one on the North side shady part of the house in Phoenix years ago and it did great. They should give you plenty of color. Nick

Not to throw a wet blanket here, but 'joey said this particular spot gets afternoon sun, so it's not shady all day. Have you considered a mix of bulbs - irises, daffs, narcissus, etc. - plus summer annuals. Your gardenias are short plants and, to me, it seems you'd have a better aesthetic with plants that won't overshadow them. Prostrate rosemary might also be a good choice mixed with blanketflowers, lisianthus, upright vinca.... the list goes on. The photos in this link happen to be back east, but you get the idea. Many of the plants in her pic do well here, too.

Here is a link that might be useful: Mixed flower bed

How about some sedums.

Never seen them before. Are those sedums in the pic?