Amorphous WO3-x thin films with color characteristics tuned by the oxygen vacancies created during reactive DC sputtering

•Amorphous WO3-x is grown by DC sputtering, with x controlled by the oxygen pressure.•The most oxidized films show interband absorption at Eg = 3.37 eV and N ∼ 1015 cm−3.•For 0 < x ≤ 0.2, W5+ states increase absorption at 1.5 eV and the electron density.•For x > 0.2, W4+ states produce absorption at 3.3 eV but no higher electron density.