Many enamelist have difficulty using warm colors. There are a couple of things that help.
First start with a hard firing flux. Flux is the base coat, it is like a primer used in painting. It keeps the enamel away from the metal, as many are not compatible with the metals we use to enamel on. The hardest leaded enamel for silver is Bovano’s #3. For Gold it is #2 And for copper is # 1
Bovano flux #3 also has a blue base tint that does not turn yellow. I have noticed in some manufactures flux for silver, if accidentally fired too high will turn yellow. With a hard fire enamel flux the warm colors are less likely to come in contact with the base of fine silver, gold, copper or sterling silver. When you are firing too long or too high the base gets hot = the fine silver and holds the heat. Then the flux gets liquid and the color layer you are firing mingles with the flux coat and eventually comes in contact with the fine silver = a phlegmy looking yellowy brown= burnt enamels. So if you fire a bit lower temp, the flux will not get liquid as you only need to melt the top layer of color. Your color enamels are lower firing enamels =melts faster than your flux. So you need to choose the temperature and firing time just enough to melt the last layer you have applied.
In the technique, you will also know you are over firing, if your enamels are climbing up the walls of your cloison wires. And think about that. If your enamel is climbing up the wires that means you have less flux at the base. So now your warm color is closer to touching the fine silver and burning.
I only want to fire my warm colors a couple of times. With too many firings they burn out and get dull. To help with this I use the flux in the cells of the warm colors each time I fill the cells of my cool colors until it is at my last several layers. Also just in case I crack the piece in setting or it was dropped or damaged in the future I will have the opportunity to re-fire the jewel without my colors burning out.
There are some warm colors that have ash or blue bases and will not burn as easily. If you do test plates you can see this, such as Ninomiya H24 and N26, yellows with ash bases. Oranges also, look at the test and you can see it has an ash base, like Ninomiya N21. It is not as bright of an orange but if you put N24 first then the N21 this brightens your orange and you will not have to worry about burning.
You really have to do the test plates and analyze the colors. On my pink color plate with 14 colors I can see some have a yellow base and some have a blue base. Use the blue base first then the yellow base one to get a brighter color, in the same color family. Do color plates and use all your warm colors, one of all reds and one of all yellows, and fire several times to see which of the reds and which of the yellows burn first. Then you really know the ones that can handle more firings. It takes time but when it is all over you will be very confident of your end result.
Here are comparisons of fluxes available for fine silver to enamelist. See in the test plate below the 5 fluxes I have tested to determine which is a harder firing flux.
Left to right Ninomiya 3, Nihon Shippo G-110, Old Thompson 757, and Soyer #3. I fired at 1400 for 1 min. All the fluxes except Soyer #3 fused.
In the second Image you will see after another firing at 1400 degrees for 2 min the Soyer fused also.
You might ask why is this so important? Working with reds is why. If you have a low firing flux the red enamel is very likely to reach with the silver and turn the color of mud.