Washing your hair before dyeing removes the natural oils from your scalp. This means the dye can come into direct contact with your scalp without any barrier, increasing the risk of irritation. Therefore, it's recommended that you avoid washing your hair the day before dyeing. The natural oils produced by your scalp act as a protective layer, reducing the likelihood of allergic reactions or stinging from the dye.
In professional salons, stylists typically apply a scalp barrier before coloring to block dye penetration. Skipping a pre-wash provides an extra layer of protection.
However, if styling products are used, gently rinse hair strands but avoid washing the scalp to preserve its natural oils.