The possibility to combine two physico-chemical phenomena is studied in this work in order to attain simultaneous control of temperature and light intensity in a greenhouse: (a) isothermal heat storage in phase change materials and (b) thermochromic behavior of complex compounds dissolved in the phase change materials. Two binary mixtures melting near the ambient temperature were studied: (a) 0.925 Ca(NO3)(2) . 4.06 H2O + 0.075 CaCl2 . 6.11 H2O melting at 35.6 degrees C and (b) 0.9 CH3CONH2 + 0.1 Ca(NO3)(2) . 4 H2O, melting at 27.7 degrees C. The melting temperature, the enthalpy of fusion and the heat capacity of both mixtures were determined from DSC measurements. Both mixtures showed to be suitable solvents for the formation of consecutive cobalt(II) chloride complexes. The compositions of the complex compounds were adjusted in each mixture so that the absorption spectra in visible spectral range exhibited low absorbance at the melting temperature and a pronounced increase of the absorbance and/or change of color with the increase of temperature in the range relevant for passive solar energy receivers. The combined latent heat storage and the outstanding reversible change of the optical properties of the dissolved complex compounds with temperature is proposed to be applied in solar heated agricultural greenhouses in areas with fluctuating climate conditions. The increase of absorbance with temperature acts as an auto-regulated shading protection from overheating.