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:

Wow!! Thanks for all the help everyone! Jean001 - I suspect you're spot-on . When we rescued this tree it was almost dead. I wasn't sure it was going to pull through (no leaves, completely dry soil, and had been for a long time). We repotted it and trimmed the branches that were clearly dead. Maybe that's the root damage you're seeing? And when you say to use fertilizer (ah! which I haven't been), just grab something at OSH or Homedepot? Nothing too specific? mmeverett - thanks for the photo! Mine are definitely approaching that color, but they're pretty big. Definitely bigger than limes I'd buy in a bag at the grocery store. I took one off and opened it up... it was SWEET. Almost no bitter taste at all that I associate with a lemon or lime. Weird!

But what if the tree aleady has an ample supply of magnesium? Applying this and that in a hope of finding the answer can cause a more complicated problem than the tree already has, and usually does not cure the problem. Maywren, has already fertilized with a fertilizer that contains trace minerals. Besides a magnesium deficiency shows definite leaf patterns, and would be easily identifiable. Actually, the proper method is to do an analysis. As you live in Florida, take a soil and or leaf samples to your local county agent for testing. Then in a couple days you will certainly know what's going on with the tree and can shortly correct it. It is really almost impossible for us on the web to know what is wrong with the tree from the information given. - Millet

You could try and prune it to a smaller size maybe if you wish to keep it in a smaller pot, my key lime is quite small and it went through several flowering cycles this year. Citrus are heavy feeders so providing enough nutrients is key. I use granular citrus fertilizer on a monthly basis just topdress the soil, and when my plants are fruiting and flowering I use a water soluble fertilizer weekly. They also make water soluble citrus nutrients which can be diluted and sprayed on the leaves. I prefer to apply smaller amounts of nutrients more often. Also I recommend using organic fertilizer, non organic fertilizer will lead to salt build up in the pot. The picture is the kinds of fertilizer I use. In the background you can see some key limes in my fruit basket.