First of all, Graymatter, the shroud, for all its similarities to a photograph, IS NOT A PHOTOGRAPH! It was not made with light... so it carries no light related color information.
There is no color information in the shroud except the rusty brown of the blood stains. The image itself is monochromatic... ALL portions of the image (skin, hair, beard, etc,) are the exact same tone. In fact, they are literally like little tiny dots. The areas that appear darker are merely denser areas of dots.
One other factor is that there is a correlation of shroud to body part distance with image density (This is what gives the shroud the "encoded" information for 3-dimensions to a VP8 image analyzer); the closer the body part, the denser the image. After a certain distance, there is no image.
On the negative (which is actually the positive print) the hair appears light because it was very close to the shroud and its image is dense.