You have a point that colour perception is culturally modulated, but I'd argue that the original point still stands in the modern world (which is what's relevant to the discussion of syntax highlighting): the majority of people who look at code will sooner associate red with bad than with good, and this association has become self-sustaining (much like the widespread adoption of qwerty-like keyboards).

Your example of Japanese taxi signs using red to signal availability is an interesting but rare exception to the general rule across the industrialized world that red={warning, danger, no, stop, delete}, and green={safe, good, yes, go, maximize}. For this example, there are many counterexamples. After all, Japan uses red=stop, green-blue=go on all traffic lights, railways, shipping, and aviation, in accordance with international conventions.

Yes, colours are very much culture-dependent, but red=bad and green=good (or green-blue for cultures that don't distinguish these two colours) has been intentionally adopted across cultures for generations -- dating back to shipping and train signals, if not much earlier to verdant countrysides versus blood, fire, etc.