Final answer:
Michelangelo applied a thin, white layer of plaster over the initial violet-colored application of plaster for several reasons. This technique enhanced brightness, allowed for corrections, and ensured the colors appeared as vibrant as possible.
Step-by-step explanation:
Michelangelo applied a thin, white layer of plaster over the initial violet-colored application of plaster for several reasons. Firstly, he added an undercoat of lead white to enhance the brightness of the oil and tempera that was applied on top. This experimental technique allowed for chromatic brilliance and extraordinary precision. Additionally, the white layer of plaster helped to correct any mistakes and allowed the colors to appear as bright as possible.