The Getty has several of the most famous ancient glass pieces. Of this one they say:
The glassworker fused together glass canes in multiple colors to form a single gob. Inflating the gob resulted in vertical bands of alternating dark blue, purple, green, and white. If you look closely you’ll see the fluid gradations within each band of color—for example, a large band transitions from pure white into icy blue and then stormy blue as it begins to mix gradually with the adjacent dark blue glass. The effect is almost hypnotizing.
Like the color-band flask above, this bowl has alternating bands of color, but here a different technique achieved the striped effect. To make this bowl, the glassworker wound a trail of hot white glass around a purple glass gob. Marvering it (rolling it against a slab) fused the trail into the gob to create a smooth surface. After marvering, the glassworker partially inflated the gob into a one-part open mold, called a dip mold, whose interior had been carved to imprint the vertical ribs around the vessel’s body. The partially inflated gob was removed from the mold and fully inflated.