The 'red hair tail gradient color' refers to a hairstyle where the hair at the ends has a gradient of red colors, transitioning from the base color to the tips in shades of red. This hairstyle can be achieved through dyeing techniques like balayage or ombre, which involve painting the dye onto the hair in a way that creates a smooth transition of color. It's a popular fashion statement that adds vibrancy and dimension to the hair.