How things had changed for J.K. Rowling by the time "Harry Potter and the Order of the Phoenix" was released. When the first one came out, they changed the title from "and the Philosopher's Stone" to "and the Sorcerer's Stone" for its US release. By the time the fifth came out, she had grown so powerful that she could name a new "character" "Nymphadora" and no one could stop her.
During this series, I've been mixing and matching influences when putting the characters together. Aside from the Mary Blair style influence, I've been taking a little from the movies, a little from the book descriptions (where Ron has freckles), a pinch of original American-edition Potter illustrator Mary Grandpre (who rightly, in my opinion, places the scar in the center of the forehead), and of course my own imagination. Tonks is the farthest away from her movie appearances as hos she looked in the movies was just so distant from how I pictured the character. Of course, as a shapeshifter she could look however she wants, but I got the feeling she usually went more for 80s-style punk than 70s-influenced rocker (is it me or do the later Potter movies seem as though the Wizarding World is just now getting Muggle fashions from the 70s? Or maybe England is?).
As a shapeshifter (or whatever the Potter series word is for shapeshifter), Tonks has the extra magic ability to change her appearance at will (unlike every other wizard who can do so with potions). This comes in handy during a key moment in the books when she... the time they need her to.... uh... Well, she changes her hair color a lot anyway.