I suggest you experiment with various transparent enamels fired atop opaque to get a feel for the effect. It's a good idea to try a sample before committing to the color change on a piece you are making. You have control over your work and your sample may not appear exactly the same as one created by someone else. There are many variables such as torch or kiln use, firing time, heat intensity, mesh size of sifter, distance of sifter from piece, depth of grains on the piece, etc. that will create differences in samples. Some enamellists make and retain their own labeled samples for reference. This is likely a good idea if your variables will remain the same as when you created the samples. If you make samples, you will, of course, want to label them. Other enamellists, including me, make samples as needed immediately prior to using specific colors on pieces. I feel I have less variation from sample to piece when I do them this way. You should do what works for you.
There is a tendency for some enamels to bubble through other colored enamels particularly if the piece is fired past maturity. It doesn't mean you have done anything wrong but is simply part of the process. There are times when you may want to plan to fire longer and increase the bubble through.
Here are some examples of bubble through that occurred when I purposely fired a transparent over an opaque past maturity.