The smallpox god was colored a deep red and green, so similar things like this were used as wards and offerings to it.
A long time ago, Hachijou-jima was said to have no smallpox due to the samurai Chinzei Hachirou Tametomo defeating its smallpox god.
Although, given that they've drifted to Gensokyo, their function against smallpox must've ended a while ago, but...