I originally listed two reasons for the relative lack of diversity among roots - the substrate and roots' primary function, transporting water and nutrients. After giving it more thought, I've expanded the list to four reasons.
1. SUBSTRATE - Burrowing animals are far less diverse than animals that live aboveground; note the remarkable similarity between moles, golden moles and marsupial moles - all unrelated. The obvious reason is that they're adapted to living in soil. Roots are similarly constrained by their environment. It's hard to imagine a giant palm leaf growing underground.
2. UNIFORM ENVIRONMENT - Leaves experience light and dark, daily temperature fluctuations and even greater seasonal fluctuations. They may be pummeled by high winds, hail and fire. These variables may influence leaf evolution in many different ways. In contrast, roots grow in an environment that is always dark, relatively protected from wind, hail and fire and relatively stable temperature wise.
3. FUNCTION - Aside from anchoring plants, roots' primary function is transporting water and nutrients. The optimum shape for this is round, similar to the veins in our bodies and the plumbing in people's homes. Rectangular roots or roots shaped like maple leaves make no sense.
4. PREDATORS - A wide variety of animals feed on leaves, which may evolve defenses against predation. For example, a cactus' spines are modified leaves that are widely believed to serve as defenses against animals in search of a juicy cactus. In contrast, relatively few animals feed on roots. There are a few species that dig for roots, and some burrowing animals feed on roots, but virtually no wild primates or ungulates feed on roots.
In summary, leaves grow in an environment where they are less constrained and less protected by the surrounding substrate and are exposed to a far greater variety of environmental variables and predators.
As far as color goes, I don't see much difference; most roots are whitish or brownish, while most leaves are green. Of course, leaves of some species do change colors in the fall, when temperatures begin to decrease. But, once again, roots are relatively insulated from the cold, especially if the ground is covered with a blanket of snow.