Answer: Plumbago would do well but turns red in fall so it might be a clash with the barberry. It is also deciduous and comes up late in the spring. It is, however, a potential choice for the type of situation you are describing. Boxwood would not be a good choice here due to the dry conditions and exposed location. A lower growing dianthus or possibly creeping phlox (Phlox subulata) might meet most of your criteria, these are the only evergreens I can think of that would. Additional candidates would be the various sedums, although they are not evergreen.
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