HCl and HF are pretty caustic i agree... in chemistry they talk about "strong" and "weak" acids and bases, and "caustic" could probably be interpreted to indicate whether they protonate anything or only attack weak (covalent) bonds

acids actually work by reducing and bases work by oxidation so i think the analogy still holds

and yeah, it probably was not ... whatever that flower is, it just happens to express blue or red or purple depending on the pH of the water that seeps in from the soil

also, that got me thinking about how purple is neither red nor blue, but both