Some Waterman fountain pens come with a refillable cartridge.
By submerging the end of the cartridge into a bottle of water-soluble ink and twisting the little knob, a piston will draw the ink up into the cartridge. This way you can use whatever color of ink you want, such as a brown or gray.
Gray and brown are no longer generally available as standard disposable cartridge colors in Waterman pens. But it must be water-soluble ink formulated for fountain pens, not India ink.
You can refill any fountain pen, even if it doesn’t have one of those special refillable cartridges. A hypodermic needle works perfectly for injecting ink into spent cartridges. Here I’m refilling a Schaeffer calligraphy pen.
I got my hypodermic needle in a kit for refilling printer cartridges.
Step by step from Instructables, using different tools.