in curiosities of lotus asia, the book featuring rinnosuke, reimu gets in a fight with the other character who fans have named tokiko. reimu's clothing was damaged so she wore some of rinnosuke's while he fixed hers. tokiko ran in for a rematch to get back her books that reimu stole and commented that reimu wasn't red. here, they're continuing it with a joke about how tokiko gets her arse kicked at every turn, but with reimu tricking her in a guessing game style.
Today it's blue!HeavenlyI can't win!You're not red!!That means that tomorrow...Emperor天帝 is a fan nick of "GIL", a guy famous for his Touhou replays not involving vertical movement. Though the reason why red + blue == him is unknown.