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This is a very important and common question.
The short answer is: Yes, but it's extremely risky, more difficult, and must be approached with the utmost caution.
Here’s a detailed breakdown:
Main Risks and Challenges
A. Unknown Chemical History
It has been processed color before. Each treatment weakens the hair.
"Already dyed" means they have undergone at least one chemical process, which alters their porosity (ability to absorb moisture/color), likely unevenly.
B. Extreme Porosity and Fragility
Typically, pre-colored extensions are much more porous than untreated "virgin" hair. They can absorb dye extremely quickly, leading to color that is too dark, uneven, patchy, or fades severely after just a few washes.
Already compromised, a second coloring process can easily push the hair into being dry, brittle, tangled, and ultimately a matted, uncombable mess.
C. Unpredictable Color Results
It's very difficult to predict the final color accurately when starting from an existing color (which might already be a mix of shades), unlike with natural hair.
For example, aiming for a lighter shade requires bleaching, which could be the "final blow" for already-dyed extensions, making them prone to breakage.
If You Still Decide to Proceed, MUST Follow These Steps
A. Step 1: Assessment and Testing (DO NOT SKIP!)
Hair Quality Check: Run your fingers down a weft. If it feels dry, rough, and lacks elasticity, it's already very fragile, and re-dyeing is highly likely to fail.
Strand Test:
Cut at least two small wefts/sections from different areas (damage may be uneven).
Test your planned dye and developer on one section.
On another section, also perform a bleach test if lightening is required.
Observe: Is the color even? Does the hair feel manageable afterward, or instantly dry and brittle? Does it stretch and return, or snap easily?
B. Step 2: Follow Core Principles
Easier: Darkening > Lightening
Lightening (e.g., brown to blonde): Requires bleaching first. This is the highest-risk operation for pre-colored hair and can easily destroy it. Strongly NOT recommended for non-professionals.
Darkening (e.g., light brown to dark chestnut/black): This is the relatively "safer" option. Use a dye in your target shade or one shade deeper, with the lowest volume developer (e.g., 10 vol), and significantly reduce the processing time (e.g., leave for 15-20 minutes instead of the recommended 30).
Use Professional/Gentle Formulas: Consider using semi-permanent or acid-based colorants with conditioning properties, as they are less damaging than permanent dyes requiring high-volume developers.
Never Color While Installed: Always color the extensions while they are loose (in bundles or wefts), not attached to your head, for even application and control.
C. Step 3: Intensive Aftercare
Pre-color treatment: You can pre-treat the extensions with a deep conditioner or a protein filler (like Olaplex No. 3) to temporarily fill the hair's cuticles, slowing down and slightly evening out color absorption.
Post-color care: A deep conditioning and reparative treatment is absolutely essential immediately after coloring.
The Most Reliable Advice
Consult a Professional: Take the extensions to your stylist for assessment. They have the experience to judge the hair's condition and can do test swatches to find the safest coloring approach.
Consider Replacement: If the extensions are not highly valuable or are already in questionable condition, the most economical and safe option is often to purchase a new set of human hair extensions in your desired color. The cost of a failed coloring job can far exceed the price of new hair.
If Coloring Fails: Over-processed extensions will become severely tangled, lose their shine, be unwearable, and are essentially ruined.
Conclusion:
It is technically possible, but the probability of failure (severely damaged hair, weird color) is much higher than success. Unless dealing with very high-value extensions handled by an experienced professional, re-dyeing already-colored extensions is generally not advisable, especially if it involves lightening. Purchasing new hair is often the wiser choice.