Have you ever seen someone, and you can just tell that person dyes his or her hair? It's like a one-dimensional, opaque cast of color that sits atop the head. This typically happens because porous strands toward the ends of the hair soak up dye quickly. Over time and repeated applications, this buildup of color can tend to look flat.
To avoid that halo effect, do this instead: Start off by applying hair color only from your roots to any regrowth. Then, about five minutes before washing the dye out, pull the color through to the ends. (I like to use a wide-tooth comb to distribute.) If your hair still looks a little one-note after following this tip, break it up by painting on a lightening product for easy DIY highlights, or head to the salon for a professional foiling treatment.