Bob typically mixes various colors of "frit" (small, crushed pieces of colored glass) to put the color into his Floats. However, he also saves the little marbles that get clipped off many of those Floats while he's making them, just for the colors they contain. Floats all start as a gob of molten glass, so Bob can fuse these marbles onto that gob instead of frit, melting them in to become the colors for that Float. Since each of the marbles contains the various colors from their respective original Float, and they fuse to the gob randomly, each marble results in it's own burst - Nebula - of color for a totally unique color pattern.
Note: Lighted Bases sold separately, and are available here