I don't think Pentel makes different colored ink cartridges for them, and I'm not aware of any compatible converters. So having different colors might not be a reason to get this drop. You also lose the archival pigment ink, which is the main reason I got this brush pen over other choices.
Pentel does make colored brush pens, though. http://www.pentel.com/store/color-brush
So that could possibly be a better choice for a multi-pen drop for this reason. And there are a few portable watercolor pens out there for those who want color.
I personally bought one of these recently, and don't see a reason to have more than one unless you want to stock up on them or give them to other people.
StreakInTheSkyTake an old ink cartridge, buy a syringe with a 15-20 gauge needle, suck up ink of your choice (Or gouache, gouache is probably best as it rinses with water) and fill cartridge. Return cartridge back into pen.
Problem solved.
Pentel does make colored brush pens, though. http://www.pentel.com/store/color-brush So that could possibly be a better choice for a multi-pen drop for this reason. And there are a few portable watercolor pens out there for those who want color.
I personally bought one of these recently, and don't see a reason to have more than one unless you want to stock up on them or give them to other people.