Spectral measurements of downwelling irradiance, Ed(λ), above the surface, and of upwelling irradiance just below the surface, Eu(λ), allow computation of spectral values of the diffuse reflectance R(λ) = Eu(λ)/Ed(λ); this yields full information about the true color and brightness of the ocean. Typical results are presented and interpreted for waters very different in turbidity and phytoplankton content. Conversely, the possibility of infering the water content from R(λ) data at selected wavelengths is examined in terms of the respective number of equations and unknowns. The necessary use of assumptions and of empirical laws is emphasized.