When you're doing a portrait, many times hair will have shades of blue and even purple in it, depending on how the light is hitting it. The key to making this look natural, like it's reflecting off the hair, is to keep the colors muted and the strokes short and thin. I mixed yellow with my purple and glazed a few layers of it over it after the paint was on my canvas to get this effect. I also brought quite a lot of the brown of the rest of her hair into where the purple was.
In the video embedded below, I walk you through my journey of painting somewhat realistic hair with seemingly unrealistic colors.
Painter of portraits and wildlife