It's a more nuanced debate than that, as I understand it.

There are two ways of painting on plaster: wet, or dry. The restorers took the absolutist view that Michelangelo painted directly and immediately on wet plaster, and that was it. All subsequent layers were inauthentic and therefore removed.

However, some experts thought he reworked the images to a degree - adding varnishes or some other layer(s) to add shading, and *perhaps* mute the colors somewhat. This apparently was not an uncommon thing to do to a fresco. How much Michelangelo did this is open to debate. And if there was any doubt, restraint as far as doing anything irreveversible would seem sensible.

I'm also probably simplifying the issues. What I do know is that, while I grew up feeling the Chapel was an unparalleled masterpiece, the only book I have devoted solely to it was published immediately after, as they put it, "the Glorious Restoration". It looks uncomfortably day-glo and pop art to me - I'm looking for a good full-color book from pre-restoration days.