I agree with Pat, reds and yellows burn up very easily, a trick
to save a burnt area that has not been filled to the top with
enamel is to finish the work in all other areas then just prior
to your last firing, overlay your burnt area with gold foil, fill
with your yellow then fire. You have to be very careful not to
stone through your enamel, but it is much better than a burnt
patch. One thing I do which helps tremendously is to test new
colors, especilally those prone to burning in a test piece.
Usually I take a small scrap of fine silver, make a cloisson,
underlay with flux, fire the flux, then add my gold foil. I then
fill the cloisson with the yellow(or red or pink or orange) and
run it in and out of the kiln until the color starts to burn.
This usually works to give me and idea of the maximum firings a
color can take. Also in my experience many of the colors in the
red in yellow families will not remain true and clear if placed
directly on silver, I always put gold foil under these colors
unless they are specifically formulated for silver.
Good luck in your enamelling!