We report the use of films of poly(3-hexylthiophene-2,5-diyl) nanoparticles (P3HT-NPs) prepared with the reprecipitation method employing water as solvent in the absence of surfactants for solid-state electrochromic devices (ECDs) and prove that these displays present enhanced properties when compared to similar ECDs with thin films deposited from chloroform. Films of differently sized nanoparticles (100 to 400 nm) were prepared and spray-coated on flexible PET-ITO substrates and tested for electrochromic properties. ECDs with switching times (t(90)) of 4 s were obtained using P3HT-NPs with a diameter of 100 nm, while those built using P3HT thin film presented longer switching speeds over 13 s for reduction (bleached to colored state). Additionally, the devices were subjected to 1000 cycles using -1.5 V/1.5 V, and the displays using P3HT 100 nm NPs presented higher transmittances (Delta(T) = +/- 20%) when compared with devices with P3HT thin film due to a more efficient oxidation step. Our data show that the availability of colloidal nanoparticles made of conjugated polymers deposited from water is an environmentally sustainable strategy leading to electrochromic devices with improved properties.