Curly hair expert Ouidad says an open cuticle is the culprit
Dear Ouidad: Why does permanent hair color tend to wear off easier on curly hair, especially gray coverage, even though I am washing hair only every other day?
A: With curly hair, the cuticle is more open and more porous, which allows it to take on more color but also can allow it to fade more quickly. Color should be semi-permanent which is more delicate. Since the cuticle is already more open, you don't need a strong activator to break down the cuticle layer to accept color. Also to help seal in color, I recommend a glaze afterward to promote tone longevity and vibrancy.