Simply put, the colored glass is in a layer on top of the base glass. There's probably technically more than that since there were marbles from the 40's which had colored glass on the surface of the marble (Vitro Conquerors) but they weren't considered to be veneered.
This picture is of Marble Kings rather than Vitros, but the broken cubscout shows the layer of color veneered onto the white Marble King base.