I might be wrong, but 6/0 is meant to be used with cooler shades, example 6/0 + 6/1 or 6/2, whereas 6/00 would be used with warm shades, example 6/3 or 6/43.
Both cover 100% grey hair when used correctly.
66/0, the double base series are to be used on resistant, thick hair, which need more pigment.
Both cover 100% grey hair when used correctly.
66/0, the double base series are to be used on resistant, thick hair, which need more pigment.