Fluids with light-tunable rheological properties can be useful in a variety of applications, such as in sensors, dampers, and valves for microfluidic or MEMS devices. Currently, such fluid formulations are available only to a few research groups since they tend to be based on specialized photosensitive molecules (e.g. a photo-surfactant or polymer). We are interested in developing new photoresponsive formulations based on common, inexpensive surfactants, with the light-sensitivity introduced by adding organic moieties that are also commercially available. The first example of such fluids was reported recently (JACS 129, 1553 (2007)) and this was a micellar fluid that exhibited a 10,000-fold drop in viscosity upon irradiation with UV light. However, in this case, the viscosity could not be increased back, i.e., the effect was not reversible. Here, we have improved upon our earlier design and we report a reversible fluid that can be switched from high to low viscosity and back by irradiation with different wavelengths of light. The zero-shear viscosity can be switched by a factor of 1,000 to 10,000 in these fluids. From a mechanistic standpoint, these effects are based on changes in the sizes and shapes of wormlike micelles formed by the molecules in water. The interplay between light-induced changes in molecular geometry and micellar structure will be demonstrated using results from an array of spectroscopic, analytical and scattering techniques.