The result is quite amateurish, but it is a nice effect I'll be experimenting with more in the future.
(Pardon the bad photos...)
The tools of the trade were:
Mark Twain Fountain Pen, M nib (I flipped the nib to draw little details);
"White Forest" green ink;
"I Love You" red ink (dipping while the pen was filled with green ink, mixing in a weird grey-purple colour).
Has anyone done this using Montblanc fountain pens?