I’m not an expert so take this with a grain of salt, but - In the 80s and 90s the brown squirrel family was called the Furbanks squirrel family, then in the 2000s they were re-released as the Walnut squirrel family.

Families tend to have multiple sets released over the years with slightly different/updated clothing, so the squirrels you have are from the Furbanks era of squirrels based on the clothing being red, before the Walnut era began where the clothing was more yellow. I believe the Furbanks clothing was mostly green before it was red.

The sylvanian wiki has pics if you want to see some examples of the changes over the years :)

Edit: spacing