enamel paints "lifting' eachother off

I don't think there's a universal rule for this.  The drying time of enamel paint depends on lots of factors:  the brand of paint, the chemical makeup of the individual color, the atmospheric conditions, the amount of thinner in the paint, etc., etc.

I recall a conversation I once had with a guy who worked for a paint company.  He said that the term "dry" is actually a slight misnomer in this context; that if a coat of paint ever actually dried out completely, it would fall off the surface.

There are several things you can do to avert this problem.  One, obviously, is to let the paint dry longer.  (I would have thought 16 hours would be enough, but apparently not.)  Another is to apply the second color in as thick a form as possible.  (The more thinner in the mix, the more likely it is to soften up the previous layer of paint.)  You could also give the first layer a shot of Testor's "Dullcoat"; that's a laquer, and another layer of enamel would be unlikely to soften it.

I think the best approach, though, is to use different kinds of paint.  I happen to be a fan of acrylics; sometimes I'll put a base coat of enamel (maybe sprayed) on the plastic first, and use acrylic for the top coat.  I instinctively shy away from doing it the other way around (putting enamel on top of acrylic), on the theory that enamel's more likely to soften up acrylic than the other way around.  But in the magazines and on the web I've encountered lots of references to people having done it that way, with no ill effects.

Jtilley is essentially correct. All paints dry, literally, but only then do they cure, and curing is the critical issue. Lacquers do not cure, they simply dry, and can be readily redissolved using the proper solvent, then reapplied. Enamels (and acrylics) use a polymeric binder. In the case of enamels, this binder, once cured, resists redissolving, and cannot be re-applied. Acrylic binders are generally resistant to water, but are often vulnerable to other solvents such as alcohol.

Unfortunately, it's not quite as simple as "acrylics," "enamels," and "lacquers." Some enamels combine a polymeric binder with a lacquer (enamel lacquers). Even when the polymer is completely cured, these paints may remain more vulnerable to the solvents used to make them. These paints dry to touch much more rapidly than normal enamels, and appear cured before they really are. That could be the problem.

Some enamels develop a hydrophilic surface as they cure. In humid conditions, this greatly increases cure time. Also, enamels (and acrylics) cure much more slowly in lower temperatues.

For brush painting, the only cure is to wait longer (24-48 hours) or force cure. As JTilley suggests, apply the second coat unthinned, paint with a wet edge, and don't go over any place after it's been painted. Wait for it to cure and apply a second coat.

If airbrushing the second coat, apply multiple, very light coats. 

Thanks folks. I'll wait 48 hours and see how I make out.

Good tips and info though!