Because you can't print counterfeit currency with black and white, which is ostensibly the reason the whole thing got started.

Couldn't you replace the black ink with green ink and then print in "Black and White" mode?

My guess is that you wouldn't get the shades right. Maybe with a lot of trial-and-error, but at some point it's easier to just make some printing plates and go that route. Either way the paper is the real problem.

So you could substitute green ink in the black ink container?