The other side is lack of colourfast pigments back then. Underlayers would be cheap and colourfast. Top layers would usually be more expensive and deteriorate much more quickly.
Most colourfast pigments are minerals, and many of them are found in the natural world (purple hues being notoriously absent). Ultramarine, cadmium red, and lead white have been known since ancient times.
IAmBroom|2 months ago