:)
If its just the green ink residue that's troubling you .... there may not be any quick fix other than to replace the bladder, which is likely some sort of silicone. That's what Parker used.
The green ink residue might be from the pen having been filled with Parker Pen's greatest error ever - Penman Ink. It is vile shit and will stain everything it touches. It was intended to replace their older (1940s-1950s) Superchrome ink made specifically to work with the Parker 51. The Superchrome was a more saturated color than Parker's regular ink - Quink. The Penman was too strong and Parker eventually killed off the line in the late 90s, but some of it is surely still in circulation.
The other problematic ink is Herbin, a vegetable based ink made in France since the 1700s ... maybe even earlier. It works sorta okay in fountain pens but has a nasty habit of staining not only the innards, but the outside too. Cellulose pens (those dreamy swirly brightly colored plastics that are just captivating) are particularly susceptible. Like this vintage Parker Duofold in Lapis Lazuli Blue:

My guess is that the staining won't affect an ink color change. Since you'll be changing to black, I dare say you'll never notice the green at all.