https://www.metv.com/stories/producers-on-the-waltons-tried-making-olivia-walton-look-older

The casting description called for an actress in her early to mid-40s, with long, reddish hair. Michael Learned was the opposite.

The article describes her as "a 32-year-old actress with short blonde hair."

But here she is in her 1973 episode of Gunsmoke, with gorgeous, very long red hair (aired Sept. '73):

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So apparently she was wearing a wig on Gunsmoke, as that was a year after the debut of The Waltons.

They tried to give her kind of a frumpy look. Impossible with Michael.