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 should not paint pure stain powders over glaze
On the left is a blue stain, right is a green. Obviously the blue is melting much better, even bleeding at its edges. On the other hand, the green just sits on the surface as a dry, unmelted layer. For this type of work, stains need to be mixed into a glaze-like recipe of compatible chemistry (a melt medium) to create a good, paintable color. The blue is powerful, it would only need to comprise 5-10% of the recipe total. Its medium would need to have a stiffer melt (so the cobalt fluxes it to the desired degree of melt fluidity). The percentage of the green stain would need to be higher (10%-15% or more). It's medium would need to be fluid (over melted), the stain would then stiffen it up to give desired melt fluidity. Of course, only repeated testing would get them just right. The guidelines of the stain manufacturer for chemistry compatibility would need to be consulted also (as certain stains will not develop their color unless the medium they are in has a compatible chemistry). And, to be as paintable as possible, use 1 part of gum solution to each 2 parts of water to create the slurry.