The first time hair dye is applied it's important to cover the entire head, from root to end and leave on according to package directions. Many colourists agree that after the initial all-over dye process, only roots need to be touched up every four to six weeks.
Ensure Even Coverage
After repeatedly dying hair and only covering the regrowth, the colour on the middle and ends of hair my become brassy, lacklustre and faded. Applying dye all over the head can help freshen old colour and keep it a close match to the regrowth.
Experts agree that to prevent damage, breakage and loss of shine, only the roots should be touched up every four to six weeks as needed. To freshen old colour on ends, apply dye to whole head only for the last five minutes of the processing time to give the rest of hair a subtle boost.