Depends on what I'm doing with the dip pen.
If it's a long piece of writing (say, a multi-page letter) I'll fill a small glass vial as an inkwell and dip into that. For extended writing, my most frequently used dipping ink is Sailor Sei Boku. It isn't technically a dip pen ink, but performs very well as such. It's also my most-used ink in fountain pens, hence I always have a bottle on my desk. I don't dip directly into the bottle for fear of contamination, although Sailor inks do seem quite resistant to getting SITB.
If it's for a craft project or some other lettering task (say, making a nice card for someone) then I'll often use a combination of the glass vial inkwell and an eyedropper. I have a particular technique that I favor for making swirls of two or more colors within each penstroke. I use a dip pen with a reservoir (my favorites are the William Mitchell Round Hand nibs with the reservoir that attaches under the nib), I dip the pen into my "base" ink (usually a thinner ink, such as most dye-based FP inks), then gently use an eyedropper to add a small droplet of a different-colored ink into the back end of the nib reservoir (usually a thicker ink, such as dedicated dip pen ink, poster color, various pigment inks/paints, etc.) The resulting penstrokes will then exhibit color swirls or color shading, with veins or patches of the thicker ink color appearing on a background of the thinner ink color. I have to work fast to make it work well, though - otherwise the thicker ink will settle to the bottom of the nib reservoir, causing the thicker ink color to be almost completely dumped into the first stroke on the page - the subsequent strokes then become much more monotonous. (Reason for using a thick and a thin ink: using two inks of roughly the same thickness/consistency usually results in them mixing together in the reservoir and making a muddy color - not what I want.)
To get the effect right takes experimentation and practice, but when it works it's really quite nice. With gold or silver-flaked dip pen ink, I can get results reminiscent of the shimmering fountain pen inks that seem to have become popular in recent years.
Edited by KLscribbler, 09 March 2018 - 15:54.