Well in fact, (in my opinion), it is about Heiji having told to Momiji that he'll later take her as his wife (the statement is kinda representing the letter and crimson is the color of love). Even if he does try to make it clear to Momiji that this statement was false because he in fact said ''Later, I'll be stronger'', it is still hinted he likes both of them (Kazuha and Momiji) and that it's difficult for him to make a choice.