The problem of the yellows/oranges/reds is that the tint is most of the time due to cadmium sulfide/cadmium selenide. This is the case for most of the current Thompson colours, probably older ones and other brands too. The most likely reason is that CdS and CdSe make colloids that are very efficient at absorbing light and re-emitting it at a specific colour, therefore the amount of material needed is small (about 0.5% per weight) and the cost acceptable. It's probable that in contact with a wire these colloids grow by absorbing copper oxide/silver oxide, and their emission wavelength shifts towards the infrared, appearing black. Tombak 15/85 probably doesn't oxidize as fast and the effect is thus less pronounced. This said, I wonder what brand, if any, sells enamels in these tones pigmented with something else than CdS/CdSe (gold nanoparticles for instance, I think the Thompson 1810 is one case, but it's yellow).
Since I am using fine silver wires, the copper must be leaching up through the 1010 base?
Perhaps I need to use a fine silver base if I plan on using oranges. Problem there is I will often get a green tint in my pinks!
It would be awesome if there was a chart of colors to use in different circumstances. After all of the years folks have been enameling, I am surprised one does not exist.
Then it reacts with silver oxide.
There's some sort of that kind of chart here for Schauer enamels http://www.emaille-hiller.de/download/Farbmuster.pdf. Here "on fondant only" means that the enamel will react in an unexpected way with the corresponding metal. For other brands maybe others can tell.