Originally Posted by
Perito
The main reason for waiting is that the scalp can get burned. The hair, itself, really doesn't care. The hair may feel dry, but that's just lack of oil in the hair. The lack of oil may temporarily make the hair more brittle, but while the hair is wet, it's well lubricated so that's not an issue.
Damage that is done to the backbone of the hair -- the chains of amino acids -- can't be repaired. Once those chemical bonds are broken, they stay broken. The biological reactions that repair cells don't occur in the hair (no blood supply, no "active" repair mechanism). You aren't going to damage the hair, per se, more by coloring it today than by coloring it tomorrow or in a week.
What I'm surprised at is the number of people who dye their hair black and then have black remorse. I never felt like black was that great a color anyway. They used to say, "If you dye it black, you can't go back." Actually, some may still say that. While I'm thinking of it, why do most people want to go from black to blonde? Aren't there any decent colors in-between. (A lot of people do try to bleach their dyed-black hair and when the bleach turns the black pigment to orange and it won't lighten any further, they are forced to dye it brown).
Now, back to the scalp. It doesn't take much to irritate the scalp. Peroxide lightly burns the surface of the scalp. Some people have "dandruff" a few days after coloring. That's not real dandruff, it's the damaged cells flaking off. If you continue to beat on a lightly burned scalp, you can develop first degree burns. Second degree burns are also claimed. I wouldn't believe reports of third degree burns. Nonetheless, first degree burns can be plenty painful so people need to think about their scalps if they want to re-dye their hair "today"!