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It may be easier for you to slap the color on and forget about it, but your hair pays the price. Every time you overlap an old color job with a new color job, you set your hair up for damage, breakage and all sorts of nastiness. A color retouch is just that -- a retouch. You don't have to color the whole thing, just the roots. But getting that color to stay put is easier said than done.
Separate your hair into four equal panels. Pin each panel up with hair clips so that you can see the roots that need to be colored around the edges and around your hairline. In other words, pin it up tight.
Use creme color and creme developer -- they're thicker and won't drip or run. Mix up your color in a color bowl and use a color brush for application.
Leave your hair pinned up for now. Brush the color on the roots that are showing around your hairline and at the border of each panel. Brush the color on from the root to the old-color line. Do not overlap the color -- not even a smidgen.
Remove the clips; the dye will keep your hair separated into panels.
Put color on your roots one panel at a time. Use the handle of the color brush to lift up 1/2 inch, horizontal sections in each panel, starting at the top and working your way down. Brush color on the top and underside of each section. Again, don't overlap the color.