These are DISR spectra of the atmosphere of Titan (i.e. changes in the amount of light with colour or wavelength). The light intensity measured by the visible spectrometer looking upward in the direction opposite the Sun increases as altitude decreases. This change in brightness is due to the increase of the number of aerosol particles, scattering the sunlight, above the probe as it descends through the atmosphere.
Visible and infrared spectra recorded in various directions provide information on the number density and properties of the 'haze' particles.