This shows the result of sprinkling a mustard colored ceramic pigment ( Thompson enamels ) on an offwhite enamel that had already been fired and ground ( to give a smooth flat matt surface as is my preference ). The whole lot was refired ( 850 C | 1562 F ) for a few minutes and then reground.
The surface does appear to have been pigmented but I know from other examples that this is "skin" deep and that a few strokes of a diamond file would remove it.
I have been experimenting with mixing the pigment into the enamel very finely but the result in most cases is that the pigment floats to the surface during the firing and I get an unattractive mottled appearance with very little evidence of color bleeding or blending.
I believe that the ceramic pigments are fritted ( perhaps because this is considered safer than raw metal oxides ).