I searched for "green and purple metal" and found this
reddit post asking what this treatment is called:
People argued whether it was anodization of titanium nitride, but someone said it was "Neo chrome" and I found these:
The description just says it is polished:
Silverware Set,20-Piece Stainless Steel Flatware Set,Tableware Set,Dinnerware Set Service for 4 (Rainbow Multicolor)
Someone commented "You can do this by heating metal and putting poppyseed oil on it."
Hmmm... heat and oil...