Preventing color build-up on ends?

I color my hair every four weeks with Clairol's Natural Instincts (a demi-permanent color), and am having a problem with the color building up on the ends of my hair. My natural color is light/medium brown with quite a bit of gray and I use a light brown or light golden brown shade on dry hair.  I like the way the color "blends" my gray instead of completely covering it (looks almost like highlights!)--but the ends keep getting darker each month due to the color building up on them. My haircutter says I should first do the roots only, then pull the color through the rest of my hair for the last five minutes, but I can't seem to do this successfully. In spite of my efforts, the dye always seems to end up covering all my hair.

Mix some luke warm water to the remainder of your colour and apply to the ends of your hair the last 5 minutes, this will only refresh your ends, not penetrate the hair.

Thanks for the replies. I know I *should* do only the roots, but I don't seem to be able to do this--the color always ends up all over the ends also. I guess I'm just not coordinated enough to do root touchups (which is one of the reasons I switched to semipermanent color).