At this point, just following where the facts are leading me, hong refers to flowers of spring or falling leaves of autumn.
The point being that this punch knocks you into unconsciousness.
General Qi has several sentences which hint at this such as.
Overtake him with a palm and he'll see a sky full of stars,
趕上一掌滿天星
When struck he is surely in danger of losing consciousness.
他受打必定昏危。
There is nothing in General Qi's writings to hint that red means the flowers of spring or falling leaves of autumn, but within the oldest books of mantis, from before the time of Liang Xuexiang, it is repeatedly emphasized that the described strikes will knock the opponent into unconsciousness and when they wake up it will be a new season- the idea being that they were out for a long while.
Here is one example:
I immediately pull back my hand and return to snatch the yue point attacking that which is unprepared. Injuring the opponent’s yue point he bends his waist, I attack with the head immediately charging my head into his face and knocking him into the ‘red peaches of next spring.’