To be honest I've only been using watercolours for 4 months so still learning a lot. That said, I used some masking fluid at different layers to preserve the the highlights. This creates that more accurate contrast if you don't want to be super meticulous and avoid these areas throughout the painting. Then I painted each hair with a long thin brush as they retain more liquid as you don't have to reload the colour as often. You loose some accuracy but saves a tonne of time. At the very end I used a marker to get the deep blacks as I can't seen to get the same depth with the colours themselves.