It turns out that adding color to plastic is a somewhat different thing than the fiber-dyeing I did for years a while back. I already knew that I could add overall color by boiling in Rit dye, but that didn't help with detail painting at all. See, Rit is both heat-activated and water based, so if you try to paint it on to plastic, it just beads up ad won't stay where you need it while you find a way to heat it up. Further research indicated that solvent-based inks tend to work on plastics when dealing with rubber-stamping and printing, so the next step was to figure out how to find or make them. Commence with the experiments! There were a few solvent inks available at craft stores, but they were expensive. I got some of the solvent used to bond acrylic pieces together, and attempted to make my own ink, but none of the coloring agents I tried would disperse in the solvent.
Then, I realized something stupidly obvious. There is already a cheap solvent ink available. It is, in fact, so ubiquitous that we often don't even think about it. It comes in many colors and is fairly easy to apply. It is, of all things, Sharpie permanent ink. A few quick experiments showed that the color, once scribbled on, could be smoothed out and made part of the plastic by a quick application of the acrylic solvent. This was a facepalm-type realization for me, since now I have a bunch of other kinds of ink, paint, and pigment that I need to figure out what to do with. I suppose I might finally take up calligraphy. :)
Anyway, I will still probably eventually upgrade to the more expensive solvent inks, since I'd really like to have something brushable, so I have more control over line, shape, and color intensity, and so it will go on smoothly from the start. As it is, I've got a good start on saving the fail-barrette by replacing the copper with a green tint.