Did Brian Keith wear a hairpiece? I have noticed that the color of his sideburns didn't match the top of his head.

Johnny Whitaker has confirmed in interviews that Brian Keith wore a toupee on the series, mentioning that the cast had to be careful not to disturb it.

I think if you'll review the pilot episode, Brian's hair looks more natural overall in that appearance -- there's some age-appropriate grey.

Based on this photo, I would say yes.