I've used Koleston too and it's very goo. I'm using Indola now as their number 4.0 is a nice dark brown with less gold tones than the equivalent in the Koleston range. I mix it with 3% (10 vol) peroxide and leave it on the system and my own hair for 20 minutes. Usually the system hair takes another go to get it dark enough but I get the system in a lighter color than my own hair. My own hair is a mid brown but I find blending a darker color easier and I also prefer the look of darker hair.