I suspect that you are seeing the effect of the flash being bounced from the non-white ceiling possibly coupled with ambient lighting. Was the room illuminated by tungsten light as well? If so it's possible you've picked up a bit of colour caused by the much redder tungsten light and this has exaggerated the effect. What mode was the camera in?

One way to minimise this is ensure you shoot in S mode (shutter priority) with the shutter set at or close to the sync speed of the camera (normally the camera will set this, or the closest possible speed dependent on lighting conditions, for you). This will probably eliminate most of the ambient light and use the flash as the sole source of light. The downside is that the background is liable to go a bit dark...

Another method is to use a custom white balance if the camera does this (see the manual for if and how) by shooting a white or grey card under the same lighting conditions and allowing the camera to calculate the colour temperature from this.

Alternatively just selectively fiddle with the colour of your hair in PS or whichever editing package you use...