Lovely pot!! To some degree, what you choose will depend on what shape you want - tall and rigid, tall and flowing, short and round, short and weeping, etc. My two favorite choices would be: Spirea Ogon AKA mellow yellow, which is fountain shaped rather than the rounded or bun shape of many spireas. White flowers, fine willowy yellow to chartreuse summer foliage, and red-gold fall color. Clematis Stolwijk Gold, which has gold to chartreuse leaves and early blue flowers. Grown on a medium trellis I think this would look stunning. Ninebark I think will be too big for the pot based on my P. coppertina. I'm personally not a fan of privet and IME they get pretty huge also. Some other thoughts: Yellow oak leaf hydrangea, Little Honey, though it has a reputation for being a bit fussy. It likes at least part shade Yellow-variegated shrub dogwoods (Cornus), though they might be too large Yellow elderberries (Sambucus) might stay small enough in your pot though they get large in the ground Clumping bamboos, though I am not sure if any of the yellowish ones will be hardy enough for you. Yellow leaved evergreens, such as some of the Chamaecyparis, would have foliage year round if you will be leaving the pot outside. Some heathers have yellow to gold to orange foliage depending on the time of year and the variety. Caryopteris such as Sunshine Blue have gold foliage and late season blue flowers. You'd need to check if the flowers coordinated with the pot. Gold grasses, either variegated like some of the Miscanthus, or solid like gold Hakone grass. You might also want to consider a water garden in your pot with papyrus, though it wouldn't be hardy or yellow. Here is a link that might be useful:

It has a virus and the fruit will most likely be bitter. I would check with you local extension agent for help with the problem. As for me I never found out how but pulled up my plants.

Thanks, gardengal! I was leaning towards oxalis, then I examined the front flower which seemed off (which I now believe was damaged by insects or not fully open) and the sprawling stems had me thinking vine! I do think it is Sunset Velvet with lots of sun exposure based on the google search I did. Appreciate your help!

@MarylandHomebody that's true about the shirt, but we painted our kitchen Simply White and it looked green. We were painting over the previous owners dark green so of course at first we thought it was that. So we primed and painted again and still green. I hated it. Eventually we went with Opaline the same color we'd painted in the living and dining room. It's the palest blue/green/grey color. I love it and it completely got rid of the ugly shade of green we were seeing that I eventually figured out was a result of our green evergreens outside. I wouldn't have wanted to live with that ugly pea/yellow green and am so glad we repainted and didn't just take somebody else's opinion that it looks good when I knew inside it did not look good to us as that's ultimately what matters.