The following response comes from internationally acclaimed salon colorist, educator and product consultant, Beth Minardi. She is recognized as one of the world's leading experts in all phases of coloring. 

 Q: A client did an at-home "color wash" on top of her highlighted hair. The color grabbed at the midlengths and ends leaving them much deeper and darker. How should I fix it?

When color is deeper on the ends, a colorist should not see this as something easy to fix. Correcting this can be extremely time consuming and challenging. During your consultation, make certain the client knows this seemingly subtle color correction is a balancing act. Ending up with burned, dry, brassy midshaft and ends, or ends with a white/greenish cast can happen easily unless the colorist is careful and vigilant. This is a time-consuming correction.At Minardi Salon, we mix a "stain remover" formula designed to lighten temporary, semipermanent or demipermanent hair color. These products are gentle, and do not disturb the natural pigment residing in the cortex. The most effective and safe "stain removers," I recommend are: Get Pure by L'Avant Garde, Redken Pre-Art, Clairol's Liquid Uncolor and Malibu 2000. We apply one of these products to the dark portion of the hair shaft in small sections-making sure to thoroughly saturate the too-dark portion of the strand. Then, cover the hair with a plastic cap and process under a pre-heated medium dryer for 15 minutes. When processing is complete, rinse, shampoo, condition and dry the hair. Refrain from evaluating the color result until the hair is completely dry and smooth. Iron the strands before evaluating the hair under proper light.You may have obtained a wonderful correction-a hair shaft more even in tone, not too dark on the ends. If however, you still have a dark shadow, then you and the client need to decide whether to proceed with a more involved corrective process or simply trim off the dark hair.If you forge ahead, you need foil, lightener and an acidic toner like Redken Shades E.Q. or Paul Mitchell Color Shines. Begin at the sides, above the ear. Weave tiny strands of hair and place on foil. Look closely at the section. Where does the hair  start to get darker? Paint the lightener formula on those mids and ends of the hair shaft in an attempt to erase the too-dark area. Wrap six to 10 foils, then go back and check the first and second foils. You may see the shade has lifted just enough. Take the client to the sink and rinse out first corrected strands. This is a lot of work because you will correct both sides of the head this way. Wrap 10, check, rinse five or 10 foils. Then, wrap 10 more foils, check again, rinse, etc. until both sides of the head are completed. These lighter strands moving through the midshaft and ends of the hair will give the impression the hair is more even-toned. You might need to softly glaze the hair with the acidic demipermanent formula, but you should only lighten the hair enough to blend-so it no longer looks as dark as the other non-foiled hair. The goal is a soft, shade-on-shade lightening rather than bleaching to an orange, rust or gold.This sophisticated, time-consuming technique works and will enhance your credibility as a color artist. You must charge accordingly and allow yourself several hours of uninterrupted time.Make sure the client uses the correct shampoo and conditioner at home, and let her know that she may need additional foiling and/or toning in three to six weeks for which there will be a charge.