In this talk, I will first briefly present the Legacy of Planck regarding the dust polarized emission (Planck 2018 XII), and a dust model (Compiegne+2011) revised to fit Planck data (Guillet+2018). I will then present new results about the SED of dust polarized emission in molecular regions as measured by Planck HFI between 353 and 100 GHz (resp. 850 µm and 3 mm). The spectral energy distribution of dust polarized emission has been characterized by Planck for the high-latitude diffuse ISM, of particular interest for the hunt of CMB B-modes (Planck Int. Results XXII, Planck 2018 XI), but not yet for the dense ISM. We correlate the dust temperature measured from Planck intensity maps at high frequencies, with color ratios of polarized emission between Planck submillimeter channels, for those lines of sight passing through molecular regions. Through this analysis in correlation, we are able to compare the temperature of aligned grains with the temperature of all grains (aligned and not aligned), and characterize the variations of the spectral index of dust polarized emission with the dust temperature. I will confront our results with current dust models, and discuss how these new constraints on dust properties compare with those already established from the study of SEDs in total intensity.