The potatoes never stay as purple as you’d like.
We have these in NZ: https://www.tepapa.govt.nz/discover-collections/read-watch-play/maori/maori-potatoes (last image on the page).
They start out the bright royal purple in the image, but by the end of boiling they turn a blueish-grey colour, and the water will be a delightful maroon-crimson shade.
If you want it to look like you’ve blended up Barney the Dinosaur, you’re going to be colouring them.
Oh yeah, great purple colour when baked or fried.
Boiled, not so much ;)