Color and detail on low poly objects are traditionally handled by various image-based texture maps, normal maps, and/or displacement maps. For an extremely low poly leaf, there may also be a transparency map involved to shape the edges of the leaf with an image, rather than using geometry to shape it.
You can create all of these in Zbrush, but they will require the creation of UVs.
Mind, these are typically functions used when exporting for render in another program, unless trying to achieve a special effect . ZBrush seldom requires internal texture mapping, having the full use of its polygon and polypaint capacity.
Luckily, Keyshot 7 can handle reasonably dense meshes, often eliminating the need for a displacement map to change the silhouette. Surface detail can be handled by a normal map. I believe they added displacement rendering for Keyshot 8, though Im not certain.