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Every good home hair colorist needs to know her head from her ash. You can't just slap an ash hair color on, cross your fingers, and pray for the right blonde to show up. Ash hair color has a specific role in the world of hair coloring, and you're only allowed to let ash come out and play under certain circumstances or you risk giving yourself a seriously fugly hair color.
About Ash Hair Color
The "ash" in ash hair color refers to the underlying pigments in this particular color. Now, all hair colors have underlying pigments that make them the fabulous shades that they are. In the case of ash hair color, the underlying pigments are blue and green. Sounds totally weird, doesn't it? But there's a reason for those strange colors -- it's to counteract the appearance of warmer undertones, such as red and gold.
About Regular Hair Color
For the moment we'll assume that "regular" hair color is any hair color that isn't an ash-based hair color. Every non-ash hair color is going to have an underlying pigment of either red, orange or yellow. Now, it doesn't mean that if your hair color has a red underlying pigment that you're going to have flaming red hair. In fact, since red is the underlying pigment for black and very-dark-brown hair, you won't even see the red at all, but it's there to add a little warmth and make your hair color look less flat. The lighter the hair color, the lighter the underlying pigment. It's always there, in the background, jazzing up your hue and making it look natural, with a bit of depth to it.
When to Use Ash Hair Color
So just when do you put a color with a green/blue base on your hair besides Halloween? Well, scary hair aside, you'll want to use an ash-based hair color when you want to control or eliminate the appearance of red and gold in your hair color. For example, if you have a Level 2 hair color (underlying pigment of red) and you want to color it medium brown (Level 5), but you'll throw a fit if you see so much as a hint of red in that color, you'd use a Level 5 ash-based hair color. The blue and green base in the ash color will cancel out any reddish-orange pigments that might try to make an appearance at your color party.
When Not to Use Ash Hair Color
Just as there's a perfect time for ash, there's also times when you want to avoid it like the plague. One of those times is if you have super-light blonde hair and you're trying to color it a little bit darker. If you put a green/blue-based color on hair that has no red or orange pigment to counteract it, you'll end up with green/blue tinted hair. Think of it this way -- just as ash controls reds and oranges, if there's no red or orange to control, ash will just go crazy and show up all over the place. So, no red or gold to control means you don't need the ash-based color.