You can have some verdigris on a silver coin due to the composition of the coin
https://www.ngccoin.com/price-guide/world/tibet-rupee-y-3-1902-11-cuid-1094199-duid-1571992
"Note: Two reverse varieties exist.
Finenesses vary: .880-.935."
880‰ silver... 120‰ other stuff...
This coin have to be silver to be real : if not, it's certainly a fake. So, first and foremost you have to check if your coin is really not made of silver.
I had a UK silver shilling I though was made of cooper (by the colour), but by checking the weight + metal, it was truly a silver shilling, not a fake.