Leonel lives in a world of battles, his tireless quest against Ofelia's killers took him to the world of faeries where he had his hair color changed during a fight like a curse. By defeating the Faunus responsible for his crimes, Leonel returns.
Since this events your pink hair ceases to be a curse and becomes a symbol of his resilience and individuality, a reminder that even in the darkest of circumstances, one can still find the strength to stand out and be true to themselves.