...to light brown, then to dark brown? It happened sort of gradually, but it's still very noticeable. Dwayne Hickman's hair was naturally dark and I guess, all of that bleach was damaging his hair, so he was allowed to revert to his natural shade.

I thought I had read this (Wiki?)—

Dwayne Hickman’s previous sitcom “The Bob Cummings Show” a.k.a. “Love That Bob” was still ubiquitous in reruns at the time, and with Dwayne’s dark hair on that sitcom, producers wanted to distinguish his persona for his new series. He was still young and popular at the time.

As time went on, he was allowed to get back to his real hair color.

cd