Each color is a separate torching unless you get to the point of trusting your firing technique enough to know what the colors will look like if they combine. I do it both ways now, but I still like the randomness of the color results. I use an eco-friendly, safe-for-your-skin pickle: vinegar and salt. I let the pieces bathe in the solution for several hours. The fire scale lifts off and the surfaces can be cleaned further with a green scrubby pad if needed. Although many artists counter-enamel the backs of their enameled pieces (torching a layer of enamel on the underside) but I just cannot get myself to do this! I love the look of the bright shiny copper after the surfaces are cleaned. So… I have found that a layer of a quality two part acrylic de-stresses the enamel of the opposite side. No cracking or slivering off of the enameled surface (ask me how I know about this :)) I love the finished pieces that have the bright copper on the back. It just looks nicer this way.
Here are some assembled pairs from this group of work: