I have used Thompson enamels for years. They come in powdered format and are sifted like glass powders but the colors are much more saturated.
Here are some examples of my work with Thompson:
Keep in mind that when you pain with glass powder you are using the exact same material as sheet glass and that very thin layers tend not to have much color in them — think about a balloon that is inflated until the latex gets so thin that the color fades.
Helios Kiln Glass Studio