That's because Kepler-186f orbits a red star, and we're just speculating that perhaps the plants there, if there is vegetation that does photosynthesis, it has different pigments and looks red.
ケプラー186f は 赤い恒星を回っているので もし この惑星に 光合成をする植物があれば 色素が違い赤く見えるかもしれません