For white flowers, try Sedum album . In poor soils, this particular species usually displays a pleasant bronze foliage. In a large rock garden, you may want to consider some of the summer shrubs such as cinquefoil, prostrate junipers and littleleaf peashrub. The latter may be in short supply locally. Before creating a rock garden, consider the time you'll have to maintain it.
Rock gardens can be labor intensive, with weed control the biggest problem.
For a fact sheet about rock gardens, contact your local office of Colorado State University Cooperative Extension. Photograph courtesy of Duncan McAlpine , back yard gardener.