Some option

I would also include low grasses like Schizachyriums and Pennisetums (sterile cultivars only).

Thank you for the suggestions! I have a few quick questions:

Do you have other recommendations for chartreuse plants? I didn’t see any labeled in the layout. Also, I’d like to avoid Dwarf Mugo Pine since it gets quite large—I’m aiming to keep everything under two feet. Any alternatives you'd recommend?

Also,I was hoping for a more balanced distribution of color. The layout drawing seems to concentrate color in the center, with mostly greens on the edges. I often see suggestions to plant in groupings of three or five, so I’d love to see a more even mix throughout the bed.

Heucheras are small, you can easily add more. Add some evergreen pink with creeping phox. It's also small, can be sprinkled in the corners. (Phlox subulata will look a little brown and dry if you have very cold winds with no snow cover, it comes back promptly in the spring).

That said, I'd include some purple ninebarks.

There are dwarf cultivars of ninebark, like the dark purple Tiny Wine or the chartreuse Tiny Wine Gold. Anna's magic ball arborvitae is stays small and is yellow. I'd go to your local nursery or online source. Chartreuse in perennials tends to be found in sedums, like Lemon Coral, hostas, like Stained Glass and heucheras, like Lemon Supreme.

There are plenty of arborvitaes that stay small. Tater tot, Anna's magic ball Little Gem Spruce is small and there are plenty of dwarf broadleaved evergreens, like Elf Mt. Laurel.