Barberries are very drought tolerant. Not only are the colors of these plants complimentary, but the different textures and foliage sizes and shape provide contrast and interest. Yellow was chosen because it is very attractive with the red barberry and green junipers, as well as the blue green Lyme grass. The soft fern like foliage of the Yarrow will provide an attractive contrast to the scale-like junipers and the weeping Lyme grass. If not cut off in the fall, the dried flower heads will be attractive in the winter.
Lavender loves sandy soil.
In fact a lavender grower told me she uses sand as a mulch. Lavender is also extremely drought tolerant. Lavender loves to be planted near a stone wall that reflects heat in the summer. Its erect form will contrast with the weeping Lyme grass. The purple flowers will also look attractive with the blue-green, red and yellow of the other plants in this design.