If you record very slow Chinese, and display the pitch (fundamental frequency) curve in a speech analysis program, you get shapes that look more or less like the diacritics themselves. So, 4th tone starts at a high pitch, and goes down to a low pitch, just like th shape of a grave accent. 1st tone has a flat pitch, just like the macron ¯. This is a design feature of the diacritic system of HP (and bopomofo, WG etc).[/quote]
Well, first of all, the third tone that emerges in very slow Chinese is not representative of most of the third tones in the spoken language, so thatʻs a problem. Itʻs really pretty much a quick low tone, not a dipping low tone with a distinct drop and rise.
But aside from that – the fact is that English does not make use of diacritical marks of this sort in a way that is crucial to the meaning of words, so it is often going to be an afterthought at best to students. Yes, itʻs shaped like the tone. But if it can be easily left off, scratched over and replaced, itʻs not expressing the way that tone is an intrinsic part of the syllable in Chinese. And if you do tonal spelling and change the spellings or insert silent letters, youʻre not expressing the way that tones can vary without the phonetic (segmental) form of the syllable changing. Color would really be the “purest” way to express it (although obviously not feasible for printing and so on).