i've worked a significant time of my life in kitchens, never came i across purple mashed potato.
i've seen it in hells kitchen, i guess, and assumed, that they use normal potatos and add some "color"
now i had to search and found purple potatoes. you said your's were yellow. this means your potatoes were partly purple?
sorry, stupid german here.
gonna leave one of my favorite foods here, kinda looks like your mashed potatoes. labskaus:
https://mybestgermanrecipes.com/north-german-labskaus-recipe/