If you live in a hot, arid environment and don’t want to lavish water on your outdoor plants, your best bet is to try xeriscaping—that is, to plant heat- and drought-adapted plants in your yard and garden. That doesn’t mean that you have to depend completely on cacti and similar succulents, though they can be extremely attractive. In fact, the flowers of some species, such as prickly pear, are vibrantly colored. You can also eat the fruit and even the large cactus leaves, or “pads,” if you dare.
You can actually get your hands on some lovely true flowers that thrive in hot, dry environments. A good example is the daisy-like perennial flower called the Gazania or Treasure Flower, which is native to southern Africa. Bluestar (Amsonia), a perennial native to the Ouachita Mountains of Arkansas that offers clusters of blue-gray flowers, is similarly drought- and heat-tolerant. Other heat-tolerant flowers include bearded iris, heliopsis, Oriental lilies, and purple salvia.
On the annuals side of the scale, vinca (a.k.a. periwinkle in some circles) is an easy flower to grow even in hot weather. Blue salvia, dusty millers, zinnias, sunflowers (of course), ageratum, cleome, and wax begonias also work well in hot climates, as do lantanas. Be cautious with the lantanas if you live in an area with warm winters; given enough water and warm weather, they can grow into tough shrubs that’ll try to take over your garden. On the plus side, their berries are edible when ripe (if a bit toxic when green), and the wood makes excellent wickerwork.
Incidentally, if your goal is mainly to add color besides green and gray to your xeriscape, consider using colorful grasses as well. For example, there are several red switchgrasses that are tinged with a delightful burgundy hue from mid-summer on.