Why it's generally discouraged:
* Unpredictable Results: Professional hair dye is formulated with specific knowledge of color theory and your hair's porosity. Box dyes are more of a one-size-fits-all approach. Applying box dye over professionally dyed hair can lead to unexpected and potentially undesirable color changes, brassiness, muddiness, or uneven results.
* Damage: Both box dye and professional dye can be damaging, but box dye tends to be more harsh. Overlapping chemical processes can lead to dryness, breakage, and overall compromised hair health.
* Color Correction Difficulties: If you're unhappy with the results of the box dye, fixing it can be more difficult and expensive than if your hair had been originally dyed with box dye or dye at home. Professional colorists may need to perform multiple corrective treatments.
* Metallics: Some box dyes can contain metallic salts. These can cause serious reactions when combined with other chemical treatments, causing breakage, smoking hair, or even burning.
* Overlapping Color: Overlapping color repeatedly can also lead to color banding, where sections of hair are different colors.
When it *might* be okay (with precautions):
* Similar Shades: If you're using a box dye that's very close to your current color and you're only trying to touch up roots or slightly refresh the color, the risk is lower. However, even then, proceed with caution.
* Fading: If your professional color has faded significantly and you're just looking for a temporary boost, a semi-permanent box dye in a similar shade might be okay. However, test a strand first.
* Temporary Color: If the goal is to add a temporary fashion color (ex: pink, blue, purple) and the initial professionally dyed hair is a light shade, a temporary box dye might work fine without long-term damage.
* If it's been a long time since your hair was professionally dyed. If it's been a long time and your roots have grown out significantly, the remaining professional dye will only have minimal impact.
Before you proceed:
1. Strand Test: ALWAYS do a strand test before applying box dye to your entire head. Apply the dye to a small, inconspicuous section of your hair (preferably underneath) to see how the color develops. This is crucial to avoid surprises.
2. Consider a Color-Depositing Conditioner/Mask: If you just want to refresh the tone, a color depositing conditioner or mask in a similar shade to your professionally done color can often be a safer alternative to a box dye. This won't lift the color but can refresh the tone.
3. Read Reviews and Research: Research the specific box dye you're considering. Look for reviews from people with similar hair types and color histories.
4. Lower Developer: Use a lower developer volume (10 or 20) if the box dye allows. A lower developer is gentler on your hair.
5. Consider the root shadow/smudge effect: If you have root shadow and you are going darker, box dye might be fine.
6. Consult a Professional: The best advice is to consult with a professional hair stylist. They can assess your hair's condition, your existing color, and recommend the best course of action. They can also explain the potential risks and offer alternative solutions.
In summary: While it's *possible* to use box dye after professional dye, it's generally risky and can lead to unpredictable and potentially damaging results. If you're unsure, it's always best to seek professional advice. They'll be able to give you a much more accurate assessment and help you achieve the results you want safely.