Lots of good answers already.
1. you can get a vastly greater variety of bottled ink than cartridges.
2. bottled ink is usually more economical.
3. no one has EVER commented (to my face [:I] ) about my regularly ink stained fingers. (most ink staining is gone within 24 hours allowing for staining with differnt ink colors without worry of how those colors might look when mixed with previous ink stain colors [
)] )
It's pretty easy to refill cartridges. Get a $.25 syringe at your local pharmacy. You might want to blunt the syringe on your grinder so you don't poke yourself too easily and refill away.
To me, the ultimate ink mixing/testing/experimenting container is a standard 1 ml international cartridge! If you make a really ugly color at least you haven't wasted much ink to do it and as long as you keep track of the quantities/proportions used in the mix it's easy to reproduce in quantity if you want to.
So have fun, and don't be afraid to get some ink on your fingers!!