It is said there are 3 reasons why Torii gate and Japanese shrine painted vermillion(red)?

1. It is believed that vermillion color ward off evil spirits. Mercury sulfied is material of vermillion paint and this is a chemical compound of Mercury(Hg) and Sulfer(S).These are known as toxic and people would like to ward off evil spirits.                                                                                                                                                                                                       
2. The Mercury sulfied(Hg2S) is used as paint material works as wood preservative. It was generaly known among the ancient Japanese that Mercury woks as insecticide.                                                                                                                                                                             
3. Vermillion color is believed it boosts God’s power.

There are torii gate which color is not vermillion(red) in Japan, such as white, blue, and black.

Some of bronze made torii gates are not painted and able to see material color.