Cobalt oxide is chiefly responsible for the intense blue color in many stained-glass windows.
Glassmakers add tiny quantities of metallic compounds to molten glass to produce color. Cobalt compounds create a powerful blue, often called cobalt blue, even at very low concentrations. The exact result depends on the glass composition, furnace conditions, and amount of colorant used.
Other oxides create different effects. Iron can produce green, blue-green, amber, or brown tones depending on its chemical state. Manganese is associated with purple and can also be used to reduce unwanted green caused by iron. Tin oxide is chiefly known for making glass opaque or white, not blue.
The colorant is mixed into the batch before the sheet is formed, so this is different from painting or staining a finished surface. Medieval and modern glassmakers used a wide range of metallic colorants, but cobalt remains the classic source of strong blue.