I've been told by hair stylists that the red color is the largest color molecule. Human redheads tend to have coarser hair than other colors. The coarser, thicker strand is necessary to hold the red color molecule.
This is why when people who are not natural redheads color their hair red, it tends to fade out more quickly than other colors. Their hair strand cannot hold onto the larger molecule.
I have no idea if this is true or not, but it seems to make sense. You may be able to do some research to find out. |