Although you may not find it satisfactory, what sissydear said IS the answer. The base glass is obviously the foundation color for the marble, and white is a common foundation color, for obvious reasons -- color laid over white will be truer than color laid over blue, or yellow, or any other base color, and if it's not overlaid but swirled in with white, white provides a high contrast that many people find appealing.
Innumerable color combinations, with and without white, have come out of the Jabo runs sponsored by patrons. For the most part, color choices and combinations have been at the discretion of the patrons of the run, within the constraints of what is possible with the materials at hand and the glass knowledge of Dave McCullough. Some runs have produced some genuinely spectacular single marbles, as well as beautiful "famlies" or types.
I'm sorry you haven't seen any.