Here's why:
* High Lift & Damage: 30 volume is a high-lift developer. On dark brown hair, it's more likely to achieve blonde highlights than a lower volume like 10 or 20. However, it also causes significantly more damage. It aggressively opens the hair cuticle to deposit color, stripping the hair's natural pigment and leaving it dry, brittle, and prone to breakage.
* Potential for Orange/Brassiness: Dark brown hair has warm undertones (red, orange, yellow). Using 30 volume might lift the hair quickly, but it might not lift *evenly* or *enough* to completely get rid of those warm tones, resulting in brassy or orange highlights, especially if you're not experienced.
* Irritation and Burns: 30 volume developer can cause scalp irritation, chemical burns, and damage to the hair if used incorrectly or left on for too long.
* Uneven Results: Achieving even, well-blended highlights requires a skilled hand and understanding of hair color theory. DIYing with 30 volume increases the risk of blotchy, uneven, or stripey highlights.
A safer approach is to:
* Go to a professional: A stylist will be able to assess your hair, choose the right developer volume, color, and application technique for your desired blonde highlight result while minimizing damage.
* Consider lower volume developer: For at-home highlights, start with 20 volume or even 10 volume developer, especially if you are inexperienced. Be prepared to do multiple sessions to achieve the desired lightness. This will be less damaging, but it might take longer to achieve the desired results.
* Use a toner: After highlighting, use a toner to neutralize any unwanted brassy or orange tones.
In short, while it's technically possible to achieve blonde highlights with 30 volume on dark brown hair, it's high risk and not recommended for DIY projects due to the potential for damage, brassiness, and burns. Consult with a professional for the best and safest results.