When the urge to mix became uncontrollable, I went down to our local pharmacy and purchased, real cheap, some little clear glass bottles with plastic caps, 2 1/4 inch high by 3/4 inch diameter. They also had bigger brown glass bottles (maybe 1oz and 2oz?) with eyedroppers.

Suggested combos: GRADUALLY mix black into green to get "Hunter Green" or "British Racing Green". I call mine "Virid Anaconda"; not the happy green green grass of home, but the color of the snake that EATS you. Add some blue to this, and you get "Scary Turquoise"; not the cheery decorator shade but the color of the ocean that DROWNS you (this is a real popular automobile color right now, for some unearthly reason).

Blue and black together proved to somewhat disappointing to me, probably because it looks like, well, storebought blue/black ink. Somewhere in there should be a killer "Midnight Blue", but it tends to go grayish in the middle stages then turn to "Weakling Black" all of a sudden.

Red and black together give you all sorts of delicious scarlets, dried blood, rust etc. STOP when you get to rust, add a couple of squirts of blue and you get a shade I call "Virgin's Blood", although I haven't researched that fully. This starts you off into all the plums, violets, cerises, etc., suffice it to say there's a whole world of purples.

A few hints:

1. Red and green mixed together make a shade called "Ooogy Mud", so keep two separate dump bottles for the reddish ends and the greenish ends. Not surprisingly, these often turn out to be the best colors.
2. COUNT YOUR EYEDROPPER-FULLS and make small batches. It's really frustrating to whip up a big winner and then be totally incompetent at duplicating it, and pouring mass quantities of good ink into some really gross aberration to try to "fix" it can be an expensive proposition.
3. You can mix and match inks from all the different companies. They're all lying, nothing bad will happen.
4. Q-Tips are useful sampling devices (they're good for Gonzo Calligraphy too), but they soak up A LOT of ink and they might put cotton hairs in your mixes. Fountain pens don't like hair. Try toothpicks instead, or even (gasp) a fine-point artist's brush. Who knows, your blotter paper might sell for millions when you're dead.
5. Keep a tip-proof bowl of water and some paper towels or rags close at hand. REAL close. Dark clothing and/or camouflage are also indicated if you're the twitchy sort.
6. Painters have been doing this stuff for centuries, so there's a lot of information available if you're not a bone-head like me who has to figure everything out myself.
7. If you're a Sheaffer-head too, spring the $5.75 for the good piston converters. They won't fit the skinniest pens, but the $4.00 squish converters don't last too long around an obsessive/compulsive ink-changer like me.

Happy trails. P.S. (Eyedroppers are also real useful for flushing out nib sections when you don't feel like cranking back and forth on the piston doohickey 42 dozen times).