Six inches of blonde hair with a half-inch of dark backing it up

“Six inches of blonde hair with a half-inch of dark backing it up.”

In the sentence above, what does “back up” mean? Is it about needing to dye the roots or something. Thanks in advance.

Yes, I think so. She dyed her hair blonde, but now the dark roots are starting to grow through. I believe this is not considered a great look.

I haven’t seen this usage of “back up”. Can someone elucidate this point, please? Thanks.

It would mean something like “following on behind”.

Thank you very much for your reply, Dozy.