How tall and wide are the arborvitae expected to grow? New plants are ALWAYS small when first planted, but that does not mean you should plant them against a wall or fence. The photo below shows what can happen when a large growing shrub or tree is planted close to a wall because it was sooo tiny when I got it!

What you should do is fill in around and between the plants you already have with perennials and annuals. Most perennials won't need attention until at least 3 years after you plant them. Choose several types of plants that bloom at different times of the year if possible. In the photo below I added tall iris under the windows, creeping phlox in front along the walkway and another white salvia to fill out the border. Finally I added a wegelia shrub in the back middle of this bed.

All of my plant samples were pink-purple, but you can find plants with yellow flowers. Visit your local nurseries as they know your climate and soil conditions best and can suggest plants most likely to succeed. I do want to caution you that winter is just around the corner, so it might be best to wait until spring to add plants.

Yes, you can use evergreens, but I think you should look for more color. Euonymus fortunei 'Emerald and Gold' has yellow edged leaves and only grows between 1 and 2 feet tall. Once again, your local nursery personnel will know more about what will work in your area.