Escaped Thoughts

I think the more important question here is, WHY are you eating purple cauliflower in the first place?!

While weighing my produce, one of the workers at the farmer's market droped a red pepper I was buying. Greatly concerned, he had me inspect said pepper for damage—it was completely unharmed, and I assured him I would buy it regardless of its brief escape. But I guess he was feeling guilty because he opened my bag and threw in a free head of purple cauliflower to make up for it. Hence, purple cauliflower for dinner!

I'll throw out a couple half-assed guesses...

It's possible the heat of the boiling water denatured the chromophore sufficiently to cause a spectral shift to the red.

It's also possible that the steaming process released compounds that changed the pH of the water used to steam the vegetable. Many plant-based chromophores are pH-sensitive (a classic example being that of red cabbage) and it seems likely, especially in light of the article describing purple cauliflower, that the same mechanism is at work here.

It should be common knowledge by now that as opalescent aphids escape being burned alive, they excrete liquid turquoise.
You did save the water.