Thanks to everyone who commented on the last post with recipe and dyeing suggestions! Devorah noted that red cabbage water can work as an acid-base indicator and so too, perhaps, purple cauliflower water. I didn't know that and had to check it out.
The Full Scientific Method would have the investigator formulate H0 and H1 hypotheses and conduct repeatable controlled trials blahblahblah. I just put a small amount of the purple cauliflower water (the cauliflower is purple, the cauliflower water is inky violet in color) in the wells of three saucers. The one on the left got a smidge of vinegar; the one on the right got a smidge of baking soda. The one in the middle was my unaltered control sample. Science!
The vinegar saucer immediately turned a familiar bright pink, the color of pickled red cabbage juice. The baking soda saucer slowly turned a green which improved in brightness and clarity over about two hours. I've read that a weaker acid (lemon juice?) would yield purple and a stronger base (lye?) would yield yellow. Alas, I have neither handy at the moment. The FSM would have the investigator note avenues for future investigation – I'm gonna pass on that.
It turns out there's lots of information available on dyeing Easter eggs and also fiber with red cabbage water (also with onion skins, paprika, tumeric, etc), which I surmise will work with purple cauliflower water too. There are so many possibilities for Saturday's dyeing session that I may need more fiber! Hm.