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

BLUE POPPY is Cone 7 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:

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.

Dry glazes need to be dissolved in water to achieve liquid, ready for use glazes.

What are stoneware glazes, what is antiquing, and other Q/A for painting and glazing can be found in Wikipedia in the section: Paint and Glaze.

Quantity: 40 g. (1,41 oz.) 
Firing temperature: 1200°C - 1250°C (2190°F - 2270°F) (Cone 5-7)
Surface: matt
Transparency: Semitransparent
Lead content: lead-free, 0%