I've used vinyl dye before, and it's pretty cool stuff. It's possible to get it in a liquid form, but it's extremely expensive and even then, it's intended for use in an airbrush. I learned this the hard way when I obtained a can of liquid black vinyl dye and tried dunking a piece in it for a minute or two. Big mistake: it melted the piece (a flexible plastic coat from one of those British 3 3/4" Harry Potter figures). I'm not sure what the problem was, chemically-speaking, but the results sucked.
For use on figures, it works much better than spray paint, provided you're OK with a much more limited color palette. It can be used on and around the injection-molded joints on the newer-style Joe figures, which is a huge plus. Because it's a dye and thus inherently a bit thinner than regular spray paint, there's a much lower risk of getting too thick a coat. I've painted over it with Testors Model Masters Acrylics, and the paint took just fine. I can't speak to whether or not it bleeds or discolors the paint, because in each case I basically paint-matched the acrylic to the dye to minimize the possibility of bleeding. The first figures I used it on (seen
here) were made at least two years ago, and while I feel like there's been a very, very slight fading on some of the softer plastic parts like the lower legs and arms, it's barely noticeable and might just be something I hadn't noticed to begin with. The vinyl dye takes equally well to both the hard and softer plastic parts on your average modern Joe figure. I haven't tried spraying it on a piece that's molded in a darker color than the dye, so I don't know how it would work in that case; I suspect it wouldn't be as effective, but I could be wrong.
I just bought a can of black vinyl dye in spray form (for some reason, the stores that carry it in this area don't carry it in black, so I had to order it), and I'm looking forward to trying it. It's definitely much more resistant to chipping and rubbing (as you'd expect from a dye), so if nothing else, it's a durable way to color a figure or (presumably) vehicle.
A word of caution: the two companies I know to produce vinyl dye also produce a similar but somewhat less effective product called vinyl paint. The cans for both varieties look identical aside from the product name, and they're almost always displayed together. Most employees don't know the difference and will lead you astray if you call around asking for vinyl dye. Take a close look at the can before you buy it. The vinyl paint works pretty well, but is vulnerable to rubbing and chipping (albeit much less than model paints or regular sprays).
