Is it a sin for a woman to dye her hair from natural black to another color such as red, blue, green, etc?
"Your adornment must not be merely external -- braiding the hair, and wearing gold jewelry, or putting on dresses; but let it be the hidden person of the heart, with the imperishable quality of a gentle and quiet spirit, which is precious in the sight of God" (I Peter 3:3-4).
"Likewise, I want women to adorn themselves with proper clothing, modestly and discreetly, not with braided hair and gold or pearls or costly garments, but rather by means of good works, as is proper for women making a claim to godliness" (I Timothy 2:9-10).
The point being made by both Peter and Paul is that people should notice a Christian woman because of who she is and not because of how she looks. Unusual hair color would violate this principle of modesty.
Neither of these passages say that it is sinful. When you have a "not" followed by a "but" where a common verb is shared but not always stated, then a Greek speaker means that what follows the "not" is insignificant when compared to the importance of what follows the "but." A woman can have braided hair, jewelry or an expensive dress but these things are far less important than displaying a godly spirit. Wearing expensive hair styles, jewelry, or clothing that screams "Look at me! I'm rich!" is not appropriate for a Christian who should display a modest attitude.