Vitreous enamel (sometimes called porcelain enamel) is typically composed of many of the same ingredients as glass. Although it can be suspended in a liquid medium, vitreous enamel's primary form is powder.
The key difference between glass powder (fine frit) and vitreous enamel is that enamel contains a significantly higher percentage of metals - the ingredients that give most glass color. The result is that enamel usually has more intense color than glass powder. While a thin layer of glass powder (once fused) might not be noticeable, even a tiny amount of enamel is likely to be visible.
It is a good thing that you only need a small amount of enamel to color the surface of your glass. The higher metal content tends to make enamels somewhat incompatible with the glass. In small amounts this isn't a problem because there is not sufficient mass in a thin layer of "almost compatible" enamel to cause harmful stress. Pile the enamel on too thick, though, and you are likely to see cracks.
