Is it just my screen?

Thanks for asking this question. From the 1950s on into the very early '70s, Beverly Garland changed her hair color often I think it's fair to say. SteveA may need to supplement and/or correct my reply. This analysis presumes no use of wigs -- but a notable exception will be clear.

Here's Beverly with blonde hair in the 1957 film Not of This Earth. IIRC in that era she was mainly a brunette as in her starring role in the cop series Decoy. Also clearly blonde in her 1960 episode of the TV western series Stagecoach West.
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In the 1968 film Pretty Poison, here we see Beverly's dark hair:
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On to M3S, here is a breakdown:
Season 10, 1969 - 1970: Bright blonde.
Season 11, 1970 - 1971: Bright blonde wig. All through the season, wig.
Season 12, 1971 - 1972: Light brunette / relatively dark blonde.

<"Hottie Alert" > In an interview from around the time she wore the S11 wig, seen in her natural hair she looks so much more beautiful. Very lovely waves:
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I haven't really studied Beverly's 1950s and 1960s looks because in my opinion, she was her hottest in the early 1970s. Gorgeous in S12 of
My Three Sons and on Marcus Welby M.D.. Her long hair circa 1971-73 is styled beautifully and complements her face so well.

Tina Cole has commented that due to the M3S shooting schedule, where episode scenes were shot using the "MacMurray Method" out-of-sequence, the actor's hair had to match from beginning of shooting to end for a given season -- so she could not add length, and in fact she had to wear a wig in Season 8 as she had short hair that grew all the while shooting wearing the wig. It's certainly possible that Beverly wore her Season 11 wig in order to make changes to her hair yet maintain the consistent S11 look. In Beverly's case, color may well have been a consideration in addition to "and/or" length.

The short answer is no, there's nothing wrong in your viewing device & you've correctly noticed a change.

Thanks for asking this question.

Yes, thanks for the question. And the answer!

Yes, thanks for the question. And the answer!

Just something about her she is so beautiful and warm. Even as a kid I thought she was gorgeous. Countered by the fact that Dodie is perhaps the most annoying character in TV history.

Dodie is perhaps the most annoying character in TV history.

Her and Cindy Brady. Gross little buggers.

Just something about her she is so beautiful and warm. Even as a kid I thought she was gorgeous. Countered by the fact that Dodie is perhaps the most annoying character in TV history.

Absolutely!