The 'Onglaze' process involves the application of liquids applied onto the fired glaze surface. These include china paints, lusters, gold, and other metallics. They are fired on at lower temperatures (e.g. cone 018).
Why you cannot paint pure stain powders over glaze
An example of why you should not just paint pure stain powders over glazes. Left is a blue stain, right is a green. Obviously the blue is melting in much better, even bleeding at its edges. On the other hand, the green just sits on the surface as a dry, unmelted layer. Stains need to be mixed into a glaze-like recipe of compatible chemistry (a melt medium). The blue is powerful, it would only need to comprise 5-10% of the total, the green 10%-15%. Overglaze recipe development projects involve following the guidelines of the stain manufacturer for chemistry compatibility and adjusting the melt to compensate for each stains melting behavior.