Do you mean in the course of the film? It looked different when it was all curled up fancy to go to market. A little darker. I thought it just had some kind of product in it.
What we got here is failure to communicate!

Didn't her hair change colour as well?

The drawing looks quite a bit like Julie's at times during the film. Julie was often working at the farm. She was a hands on boss. And yes, they probably styled her hair since they were making a movie.
What we got here is failure to communicate!

First of all, the drawing does not look like a woman running a farm, rather a woman who works in a farm.
Besides, Christie's hair was loose, not put up or even braided. For Fanny Robbins, an employee in the farm, it's passable. But for the mistress of the farm, no.
ETA: Not to forget the volume. In some scenes it is quite obvious that a teasing comb and hairspray had been at work.

Her hairstyle was also very reminiscent of the period she was portraying. A woman running a farm in the 19th century. When she went to town she had her hair done up in the style you would see in pictures, otherwise it was just let go. Like this period drawing.
http://hoocher.com/Jean_Francois_Millet/19th_CenturyFrench_Young_Woman_(Catherine_Lemaire).jpg
What we got here is failure to communicate!

The middle parting, the bangs, and the volume are all very reminiscent of the 60s, even if the hairstyle is not technically a 60s hairstyle.
http://ww3.hdnux.com/photos/24/76/23/5498102/5/628x471.jpg
http://www.manners.nl/wp-content/uploads/2014/01/Manners-Brigitte-Bardot-24.jpg

Same here! I don't get what people mean when they say it was a 60's style. They most likely read it somewhere.
What we have here is failure to communicate!

I was young in the 60's and never saw hair worn like Bathsheba did.

You can't really see much in those pics, just the bangs.
What we have here is failure to communicate!

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You're right. As they say, there's nothing new under the sun.