In Hero (2002), why was it required to use red ink?

In the movie Hero (2002), Broken Sword says:

"Eight feet for that size I will need red ink…"

What is so special about red ink, instead of using regular black ink?

According to Yilei Yuan's 2016 PhD thesis "Subtitling Chinese cinema: a case study of Zhang Yimou’s films", red ink (specifically made from cinnabar) was required for "for maintaining the colour of the calligraphy" at such a large size.

Broken Sword makes an enquiry to Nameless about the scroll dimension of calligraphy he prefers, and he replies ‘八尺’ (ba chi, eight feet of old Chinese measurement) that demands ‘朱砂’ (zhusha, cinnabar) for maintaining the colour of the calligraphy.

The literal translation appears to be:

八尺剑字需要朱砂。(Hero: 18.38)
A Chinese character of ‘sword’ of eight chi needs cinnabar.

(From Zhu Zhu's 2014 PhD thesis "Translators’ Communicative Assumptions in Subtitling Chinese Feature Films into English".)

Cinnabar is used to make the colour vermillion, which is a very bright and long lasting red. Wikipedia notes "In China, the colour vermilion played an important role in national culture. ... It was also used for unique red calligraphic ink reserved for emperors." The "Colour in Chinese Culture" page adds "Writing in red ink was traditionally exclusive to an emperor's comments added to memorials."

Outside of the story of the film, of course, the reason for the red ink is because the director wanted to divide the film into sections, each dominated by a different colour, and this scene is within the red section of the film.

Why then did Mr. Zhang pick Mr. Doyle to shoot ''Hero,'' his first attempt at a martial-arts movie with digitized action sequences in the style of ''Crouching Tiger, Hidden Dragon''? According to Mr. Zhang, it was because Mr. Doyle is known for pushing film to its limits in order to produce extraordinary hues, and Mr. Zhang's plan was to divide ''Hero'' into five sections, each dominated by a single color.

Red was the first color Mr. Zhang chose, which presented Mr. Doyle with an immediate problem: in his work with Hong Kong directors like Wong Kar-wai, Mr. Doyle had made a conscious effort not to use the color. ''Up until 'In the Mood for Love,' '' he says, ''we avoided red at all costs. I think I've probably said on at least 25 films, 'No red,' because it has too many associations in Asia.'' Then he had to find a way to produce images that would match the unusual red of the hand-dyed costumes. To do this he decided to switch from Fuji, the film brand used for most of the film, to Kodak. ''The red is a Kodak red,'' he said. ''It's a much more saturated solid red.''

(From The New York Times, "Cracking the Colour Code of 'Hero'".)