In Greece today we have people who have brown hair with a blondish or reddish tint, the sun lightens the hair too etc. And actually the word xanthos which is translated as blonde or even worse yellow could have meant something like tawny. It is likely that it was used for light brown to dirty blonde. I really doubt anything lighter than that had existed.
Why do you doubt that? There are plenty of ancient Greek depictions of people with blonde hair.