As I promised, I'm back to share the results of my mashed purple potato experiment.
This batch of purple potatoes was not as purple as the first, so already we've got an unexpected variable to contend with. This is terrible science*, but at least I can admit it - the worst terrible science is the kind that tries to pass itself off as the real thing.
I blithely assumed that the potatoes from Wegmans would look like the potatoes from the farmer's market on the inside, because they looked the same on the outside. Consider this proof that you cannot judge the purple-ity of a potato by its skin. The Wegmans potatoes were pale immediately under their skins, but a medium, radiating purple at their core. Not as dark as the first set, and with more white streaks through the middle, but definitely still purple.
I treated them exactly like I would normal potatoes. I boiled them until they were fork-tender, drained them, and then mashed them by hand with some milk, butter, salt, and pepper. The water I poured off was an unappetizing grey shade, and the end result was a sad greyish lavender color, with a few purple chunks.
Honestly? They reminded me of Montreal's slushy winter streets, and despite them tasting exactly like regular mashed potatoes, I wasn't really happy with them. Some of the color washed out during the boiling process, since the water was greyish, but adding milk likely also contributed to diluting the purple. Maybe very purple potatoes would have fared better, but I'm not going to keep trying, since they're quite a bit more expensive than regular old potatoes. Nope, it'll be russets or reds for mashing, and if I decide to get fancy with purple potatoes again, I'll roast them to preserve their beauty and impress friends.
*To make this a little more accurate, I'd have needed white potatoes to boil and mash and compare to the purple ones, because maybe the boiled-potato-water is grey for white potatoes too and I've just never noticed before because it goes straight down the sink. I only saved the boiled-purple-potato-water because I was expecting it to have color. An experiment with no control, conveniently confirming my hypothesis? I hang my head in shame. Maybe they'll let me use the spectrophotometer at work to see just how much color difference there is in the runoff from boiled white vs purple potatoes...