Since your hair has a direct impact on your appearance, coloring it can be a high-stakes decision. When it comes to dyeing your hair, there's homework to do to make the most informed decision possible. Some questions that might come to mind about hair dyeing are: Should I color my hair at home with a boxed hair dye or go to a professional? Should I ask for a refund if my hair doesn't achieve the exact color I ordered?

However, if you're new to hair coloring, whether or not you should wash your hair before dyeing it may be the least of your concerns. Until your scalp has experienced an hour or so of a burning sensation while waiting for the color to develop, you might not realize the importance of prepping your hair for coloring. Some hair dyes have skin-irritating chemicals, which cause the surface of your scalp to sting and give it an itchy rash, per Head and Shoulders. While harsh skin reactions to hair dyes are common, you can keep them to a minimum. One of the biggest favors that you can do to your scalp the day before altering your hair hue is to not wash it, L'Oréal Paris points out. Here's why.

Hair dyes contain various ingredients that, when sitting on the hair for any length of time, may cause varying degrees of skin irritation. Most allergic reactions to hair coloring products are caused by an oxidative chemical known as paraphenylenediamine (PPD), according to Dermnet NZ. The reactions, while common, can range from mild to intense depending on the sensitivity of your skin, from a brief rash on the scalp to a swollen face. If the reaction becomes severe and lingers hours after coloring, you might need medical attention. In rare cases, per National Health Service, hair colorant can trigger anaphylaxis, a potentially life-threatening allergic reaction. 

To know if you have PPD sensitivity, take a patch test by dabbing a small amount of the colorant that contains PPD behind your ear or on your inner elbow and keep an eye on the area for two to three days. Do not use the product if you have rashes or other signs of discomfort following the patch test. Instead, opt for PPD-free hair dyes. Whether you intend to color your hair by yourself or go to a hair colorist, check the product labels for irritants and allergens that might give your scalp a hard time before working the dye into your hair.

Once you've tested your sensitivity to certain ingredients in hair dyes, prep your hair for coloring. "Your hair should be dirty because the natural oils help to protect against the harsh chemicals," says hair colorist Terri Fe to Makeup.com. "The grease serves as an extra coating so that when we go to break down the cuticle, there's less damage and breakage to the hair and irritation to your scalp." So technically, dirty hair makes the burning sensation from hair dye less intense. This rule of thumb works for both virgin hair and touch-ups. Besides, since you'll get a thorough hair wash after your color treatment anyway, a skip of hair wash beforehand will save you some shampoo and conditioner.

Nonetheless, L'Oréal Professionnel artist and colorist Katie Collette suggests that there isn't an all-encompassing answer to the question of whether or not you should wash your hair before a color treatment. It all depends on the color of your choice. "Anytime you are wanting to deepen your color or add lowlights, clean hair offers the most accurate read of your natural color," Collette told Hair. For bright, colorful hair shades that need a decent amount of bleaching, you can skip a wash or two before a color treatment. Knowing what sets off your sensitivity and avoiding colorants that cause allergic reactions are crucial whether you have your hair colored at home or at a salon. It's best to discuss the color you want and your skin condition in advance with your colorist and to obtain expert advice on how to care for your hair both before and after a hair dye.