There are two (or more) kinds of nickel silver… the cheap one
without nickel and the more expensive with nickel. The last one is
hard to patinate.
If you happen to have the first one this will work: Super Blue (Gun
metal blue from the gun shop) makes it brownish (great for blackening
brass) Silver blackener (that is not lime sulphur) makes it black
grey. These are the easy beat patination processes.
NOTE: I had the most beautiful green patina from old saturated acid
powder (Hera AM99): was sold in Europe in the 80ties as an
alternative to (sulphuric acid) pickle (abbeizmittel). I have tiny
bit left and cherish it, because that green is incredible! Somebody
knows if it is still produced?