Here's a more detailed explanation:
* Color Theory: Dark hair is very pigmented (full of brown and black melanin). To see a color like turquoise, you need a lighter base for the dye to deposit on. Think of it like trying to paint a light color onto a dark canvas - the dark canvas will show through.
* Dye Formulation: Most semi-permanent and direct dyes (which turquoise often is) work by depositing color *on* the hair shaft, not by chemically altering the hair's existing pigment. They need a light base to adhere to and be visible.
* Expectations Without Lightening: Without lightening, you might get a *very subtle* tint of green or blue in direct sunlight, but it's unlikely to be a noticeable turquoise. The dark brown will overpower the turquoise.
To get a true turquoise color on dark brown hair, you'll need to lighten it first. How light you need to go depends on the specific turquoise shade you're aiming for. A lighter, pastel turquoise will require a lighter blonde base than a deeper, more saturated turquoise.
In summary, don't expect turquoise dye to show up well on dark brown hair without first lightening the hair.