As it turns out, I don't have enough memory to even finish pre-calculating the light for 140,000 strands, let alone render it (for now, at least). So I toned it down to around 70,000-80,000 hairs, and rendered with global illumination enabled. I still couldn't get that one group of hair in the back to change color. You can still see the outline of the head through the hair; I'll have to check to see if there's transparency applied to the hairs, otherwise I'll either try adding more density to those areas or making the strands themselves thicker.