If you are going for
If you are going for reflected colour, you will never get the really bright effects you get from coloured glass. Opaque paints are always duller than transparent ones, even when you fire them high enough to get a shine to the enamel. If the final piece is to be seen with transmitted light, the opaque enamels will block a large portion of that light and appear dark. If it is to be seen in reflected light, the appearance will be colourful, but possibly not bright.
So my suggestion is also to play/experiment with different firing temperatures to see the results. Try temperatures above the recommended ones to learn their effects.