Andy,

Selecting the right flowering plants for your garden wall depends on the size of the bed, the amount of sun or shade the area receives (summer and winter), whether or not the area receives supplemental irrigation, whether you want annuals or perennials, what else is growing nearby, your aesthetic, and a host of other variables.

My recommendation is take some measurements and some photos of the area to an independent garden center and consult with the staff there.  They can make some suggestions and you can view the suggestions to choose the one(s) you like best.

Both Impatiens and tuberous Begonias come in that come in many different colors and in Denver's cold hardiness zone 5 are grown as annuals.  They both will do fine growing in shade with supplemental irrigation.  Choosing one or the other is a matter of personal taste.