If you google the word “Torii”, most of the torii gates appearing are indeed red, vermillion or orange. But why are they coated in exactly these colours?
Finding a clear answer to our question is not easy, but according to the Fushimi Inari Shrine (the one with all the gates) located in Kyoto, the colour acts as a blockage or obstruction to magical powers and evil spirits. Further, at the same time as expressing the lively motion of life itself, vermillion also serves as protection against misfortune and calamity. It is typically found on ancient palaces, temples and shrines. Additionally, the vermillion colour found at Inari shrines express the abundancy of the power of Inari Okami (One of the main kami/deity of Shinto). It plays a role as an enhancer of the powers of the kami.
Another explanation to the the wide use of vermillion is that the coating used to preserve the torii contains a mineral giving off the distinctive colour (see mercury sulfide).
As many other aspects of Japan, the red colour of the torii gate is also shrouded in mystery.
Be ready for more deep diving into Japanese history and culture!