Colors in glazes are affected by the clay, slips, stains, or underglazes below them. Most ceramic colors, however, are a result of metallic oxides being dispersed in the fabric of the glaze itself. Under varying circumstances, these colorants can give very different results.
Warning
Certain metallic oxides can be dangerous if inhaled or consumed. Always exercise caution and do careful research on the metallic oxides in glazes use for functional ware.