The Gundermen (and the original Hyborians) are noted as being “tawny-haired” but I’m not sure what this means. I’ve seen it suggested that it means blond, but I’ve also heard light brown. Looking tawny up today and it talks about a light brownish-orange color (ginger?), but I don’t think this is what REH was using as his definition 80+ years ago. So what color hair are the Gundermen supposed to have (and please don’t give the non-answer of “tawny”)?
There’s no connection to hair colour, but the most memorable reference to “tawny” I know is about a kind of Port wine. Tawny Port Wine is golden-brown. Perhaps it was that tone that Robert E Howard meant at the time.
Tawny is a light brown to brownish-orange colour, from what I recall - like a lion’s mane.
Agree with Bill
More interpretations and variations 
So basically something between the colors of the mane and fur of Ookla the Mok (from Thundarr the Barbarian) then?
My wife told me it sounded like tawny was, more or less, the color of caramel.
Yes…exactly.