A pair of solution-processable electrochromic polymers poly(2,3-bis(3,4-bis(dodecyloxy)phenyl)-5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)quinoxaline) (P1) and poly(3,4-di(2-ethylhexyloxy)thiophene-co-3,4-dibutoxythiophene) (P2) were designed and synthesised through a chemical oxidative polymerization and copolymerization approach. According to colour mixing theory, the two polymers with complementary colours (green for P1 and purple-red for P2) were utilised to create black-to-transmissive switching films via co-processing from a solution containing them in an appropriate mass ratio. The films exhibit a deep black colour, moderate optical contrast and robust stability. This method and the films with favourable electrochromic performances could be employed for the realization of potential large area electrochromic device applications.