I've had Sebastian applied at both the salon and I've done it at home..my stylist suggested it to cover my gray hair.
I think everyone answered the questions for you well. I'd just add that when you have it applied after you relax, the color will last MUCH longer b/c your cuticles are open. It's not damaging, b/c a cellophane only coats the hair.
My stylist always uses one of those little bowls b/c she mixes a few colors together. I tried the same at home to good results, although I don't think I'm able to make the all over color pop quite as well as she does. I noticed after she did her mixing that over the weeks, the colors fade away and reveal themselves. So she mixes a brown based color with a red based color and a gold based color, so at the beginning my hair has a reddish-brown tinge all over, but as that fades, the gray hair will look like red highlights, and finally kind of golden. Everyone always comments on it, and wants to know what it is, so I thought I'd share that trick...I think it only will show up like that if you have some gray hairs sprinkled through your hair.
Oh, I think she has me under the dryer longer than 15 minutes and I did read that more porous hair will need more time, which I think probably describes our hair esp. after relaxing.