Most historical enamels were opaque but some were transparent. A neat Anglo-Saxon technique was to use transparent red enamel over a textured gold surface that sparkles in the light. It looks beautiful. A lot of modern enamels aren't enamel at all but specially-designed polymer substitutes like Ceramit and Colorite. We are going to use Ceramit on my reconstruction of Agamemnon's cuirass. http://www.myarmoury.com/talk/viewtopic.php?t=28063
Where the Romans did use enamels they also used many types of paste inlays such as this umbernate brooch.
In fact with this apron strap from a belt they have used both situations where the yellow is indeed enamel and is opaque however the red inlay is a paste one.
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I have often wondered this as well. This is the Celtic Battersea Shield which is an excellent example of enameled bronze. It was dated rather ambiguously (350 bc - 50 bc) and is of Insular Celtic origin so I don't know if it would apply to what you are looking for. But it appears to be opaque.
"The strong did what they could, the weak suffered what they must."
Dan, that is an interesting idea for making transparent enamel so that the gold beneath is visible. Its almost the same as in oil painting where a darker glaze is used over a lighter color to give an optical color combination that can look like a glow.
"You have to laugh at life or else what are you going to laugh at?" (Joseph Rosen)