The pH article posted by Jude was very interesting and helpful. I'm trying to figure out why semi permanent dyes are (according to the box) supposed to be applied to damp hair (e.g., Clairol Natural Instincts). I called Clairol's help line, but all they would tell me is that the colour might turn brassy if applied to dry hair. But as hard as I tried, she could not tell me *why* this would happen. I'd like to know whether I'd get into big trouble if I applied it to dry hair. Partly because it is so much easier to apply to dry hair. I'm guessing that some of the processes described in the pH article by Jude are important here, but I'm not sure why the semi and the permanent would react differently. In fact, some sites say never to apply permanent to damp hair because the colour can interact badly with chemicals in the water. So how is semi different that is HAS to be applied to damp hair?

I don't know, I always apply the semi-permanents (SFX, Manic Panic etc) I use on dry hair. I find it's not that easy to apply when my hair's damp and I get a better idea of where the colour has been applied well, when my hair is dry. And at least the semi-permanents I use, don't turn weird colours when I apply to dry hair. On the contrary, when my hair is damp it's more likely that I don't apply enough dye or miss a spot. O_o

Maybe you're supposed to apply to damp hair to get better coverage and because it's supposed to make it easier? I'm just guessing. But I have heard it too that some people apply dyes on damp hair. But not sure why.

I don't think you need to worry about water chemicals too much because they test for that sort of thing.

My guess is that it makes it easier to apply, in that the dye spreads more easily. It probably prevents the colour developing with dark spots if you apply it unevenly. Direct dyes like Special Effects or Directions don't develop like Clairol dyes, they just soak into the hair. On the other hand the type of semi-permanent you mentioned has to interact with your hair so it makes it a more complex process.

My advice is to follow their instructions. They spend millions on development and testing so I'd expect following the instructions will give you the optimal result. Hope this helps