Heating of Enamel coated cooking utensils and health hazards
I unfortunately don't have time to dig through these articles in great detail, but they should help get you started. Feel free to comment with more questions.

The overall impression I got from my brief search is that there is definitely a potential health hazard depending on how the coloring was made, but in most modern countries, there are  regulations in place to prevent those kinds of problems. Enough about what I think though, let's get into the references.

According to Wikipedia, the coating on most cookware is vitreous enamel, which is made by heating a mixture of glass powder and pigment to a very high temperature and letting it melt onto a surface (much like glazing in pottery).

The glass itself is pretty much harmless, so the main danger comes from the added pigments. Here, we're stuck, because it really does depend on what was used to make the pigment. If it's a zinc pigment, you're probably fine. If it was cadmium based, you might be kinda screwed.

I've found references to several articles which describe experiments trying to find out what the leach rate of metals from the glass into a solution is (usually by boiling in vinegar), but I can't hunt down the fulltexts right now, aside from the one open-access article from the NIH. Here's what I have:

Full text:Lead, Cadmium and Cobalt (Pb, Cd, and Co) Leaching of Glass-Clay Containers by pH Effect of Food 
While this study was on clay containers, it should theoretically apply to glass-on-metal as well. Keep in mind that the containers seem to have been acquired from local craftsmen in Mexico, so the results won't necessarily apply to stuff you find on the supermarket shelves.

Abstract: Hot leaching of ceramic and enameled cookware: collaborative study.
This is what you're looking for, but I can't find the full article, and the authors seem to have been more interested in establishing a method than finding the health effects. If you can find the full paper and cross-reference their results with acceptable limits of exposure (maybe from the EPA), that'd probably be your best bet.

The second link also has other articles on similar problems. If you can't find your information from these two articles, maybe you can find it by following the links on PubMed. Good luck!