Glass fusing is the fusing of glass (flat glass, granules, glass powder) and forming glass in a glass furnace. Glass contains metal oxides that determine the color (iron makes green, cobalt makes blue, silver makes yellow, etc.) If you melt two different colors/types of regular glass together, they will shrink differently when cooling and you will get tension in the glass, which can lead to breakage.
When you use compatible tested glass, you can fuse different colors of glass together without tension. Fenix ​​is ​​the specialist in compatible tested glass for glass fusing.