<p>Is it possible to use a standard color inkjet cartridge to color filament for full color 3D printing?</p>
<p>It seems like a natural next step to me, but I haven't seen much of anything on this. (Just a few ancient experiments on reprap wiki.)</p>
<p>I've learned that some inkjet printers have the heads built into the cartridge whereas others it's part of the printer.  I think the former would be more appropriate.</p>
<p>Unfortunately I haven't seen anything on actually how to drive the cartridges.  I'm guessing the mfgr's treat this as a trade secret (?)  Still, there's got to be some overseas reverse-engineer... something... on this, right?</p>
<p>Anybody have resources/notes they'd like to share?</p>

I don't think it makes a lot of sense - you don't need that kind of resolution, and getting a sufficient amount of ink that way to coat the filament would be hard. If you're going to be switching colors rapidly, you'd need a long purge between colors anyway. I also doubt the type of ink is suitable for sticking to filament materials.
If you really want an automated filament coloring system, I would do it with Sharpies and actuators to move individual ones on/off of the filament as it passes through. Coloring PLA with Sharpies prior to printing is a known-working technique, and there are even models available on Thingiverse for holders to keep them in place while the filament runs through. Designing the actuators to switch individual ones on/off, and the firmware controls for them, would be the natural next step.
Here are some examples I did with manual coloring of natural translucent PLA (from left to right: uncolored, silver Sharpie, and red Sharpie):
I didn't color a long enough segment of filament or properly purge for any of them, which is why the coloring is inconsistent/incomplete. But the technique definitely works.