When restorers began work on cleaning the Sistine Chapel ceiling in 1980, what did they discover had affected the original appearance of the frescoes?

It revealed a contradicting unexpected brilliant colour which hasn't been seen in a long time.

Explanation:

The colour contradicts the sculptural qualities of Michael Angelo master piece with a pressing question among the restorer whether the creator used a secco paint to modify his finished fressco painting after the plaster had dried.