"She looked the youngest there; she has a pale heart-shaped face, dark twinkling eyes, and short spiky hair that was a violent shade of violet.
...a second later, her hair had turned bubble-gum pink.
'How did you do that?' said Harry, gaping at her as she opened her eyes again.
'I'm a Metamorphmagus,' she said, looking back at her reflection and turning her head so that she could see her hair from all directions. 'It means I can change my appearance at will...'"
I try to follow canon (and I am not opposed to her movie appearance), but seeing as Tonks can make herself look anyways she wants, it's a moot point. However, my fanon version* always pegged her as a pale, ultra-skinny, 90s-grungy pixie with big, expressive eyes... Like a cross between Mila Kunis and Amuro Namie with technicolor hair.
* - also, my fanon Lupin was Edward Norton**
** - and Sirius was Brad Pitt when he had his Grizzly Adams beard
PS - Hurrah for my first GIF!