The options for color in that area are going to be seasonal. Something that blooms in one season or perhaps something with colored foliage. Most blooming shrubs get large enough to at least match the boxwoods. I might consider using a perennial such as dwarf variegated ginger (yellow and green striped foliage) or Aztec grass (white and green plant like liriope). Another option is to use annuals such as caladiums, impatiens, or torenia (is light levels are pretty good).
If light levels are pretty good (very bright shade to part sun) you might get a dwarf hydrangea, dwarf gardenia or dwarf nandina to work there. Other shrubs that stay dwarf enough are going to be basically green and thus not very good against the boxwood backdrop.
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