Why have enamel colours often remained brighter than oil paints on canvas?

Enamel colors often remain brighter than oil paints on canvas because enamel is a vitreous, glass-like substance fused to a surface, rather than a pigment suspended in a drying oil binder.12

The brilliance of enamel is derived from the use of metallic oxides incorporated into a molten flux—a compound of flint or sand, red lead, and soda or potash. Because this material is essentially a form of glass, it is inherently more resistant to environmental degradation than organic oil binders, which can yellow, crack, or fade over time when exposed to light and atmospheric conditions.1

Furthermore, the durability of the color depends on the quality of the fusion process. When properly fired, the enamel forms a stable, hard surface that withstands atmospheric agencies that would otherwise cause the decomposition or breakdown of less stable artistic mediums.1