What you need, to get permanent and water/heat-resistant colours, glazing is the right way to go. INDIGO BLUE stoneware glaze is leadless and food-safe

POWDER BLUE is Cone 9 matt finish glaze that looks like high fired reduction porcelain glazes. Color effects and other reduction surfaces are achieved without the cost or trouble of high fire reduction. It's an unusual and interesting glaze and gives the best results when applied thin and fired on flat surfaces. On vertical surfaces and thicker application, the results will vary. Artistic/special effect glazes usually require a generous application to achieve the desired effect.

Glaze preparation advice for Indigo Blue Color: 

Liquid glazes are perfect for brush on. It should be applied with a soft brush in a thin layer after the first fire. After drying please fire between 1200-1280°C. Shake and stir very well prior to use. If the glaze becomes thickened over time simply add water and stir thoroughly.

Powdered glazes, once mixed with water according to the recommended guidance, are suitable for dipping, pouring or spraying onto the piece. In some cases they are suitable for brushing-on, however, that's not easy.

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Quantity: 40 g. (1,41 oz.) 
Surface: matt
Transparency: Semitransparent
Lead content: lead-free, 0%

What is a glaze, what is antiquing and other Q/A for painting and glazing can be found in the section: Paint and Glaze.