This cone 6 porcelain bowl has a black engobe inside and half way down the outside. This inside glaze is a transparent (G2926B) but the outside is that same transparent with 11% added encapsulated red stain. Notice that the glaze is so opaque that you cannot see where the black engobe ends and the while porcelain body begins!
Opacity of ceramics glazes is normally achieved by adding an opacifier like tin oxide or zircon. However there are chemical profiles that can turn transparent glazes milky and make it cheaper to opacify them.