Courtesy of Hype Hair
In the End
The ends of the hair are much more porous than the rest of the hair shaft. The hair near the root area may process faster because your body's natural heat works to speed up the process of hair coloring. Therefore, when applying color, consider applying it at the midpoint of the hair and work the color from the midpoint up the root area and then down to the ends. If you are just doing a touch-up, you only need to process the new growth.
If your hair is both long and thick, you might have better results if you color your hair in sections.