Hair dyeing is not only a fashion statement, but also a science of color.
From developing and bleaching to locking in color, each step involves fascinating chemical principles. Many people often ask: How exactly does hair dye apply color? Why do some colors fade quickly while others last for months?
I will guide you through a comprehensive understanding of how hair dyeing works, the types of hair dyes, the levels of developers, and how to choose the most suitable hair color for you.
I. The Basic Principles of Hair Dyeing: The "Scientific Game" of Getting Pigment into the Hair Strand
To change hair color, the first step is to "open the door."
The surface of hair is covered with a layer of tiny "scales"—the cuticle—which protects the cortex inside the hair strand. During the dyeing process, the alkaline components in the hair dye slightly open the cuticle, allowing pigment molecules to penetrate into the hair shaft.
Next, the developer—usually containing hydrogen peroxide (H₂O₂)—breaks down the hair's existing melanin, making room for new dye. The dye molecules then form a new color structure within the hair shaft through a chemical reaction; this is the "coloring" process.
In simple terms: Hair dyeing = opening the hair cuticle + removing old pigment + injecting new pigment + locking in color.
II.Types of hair dye: Different types determine the depth of color.
Based on the durability and chemical strength of the dye, hair dyes are mainly divided into three categories:
1. Semi-Permanent Dye
This type of hair dye only deposits color on the surface of the hair shaft without damaging the cuticle.
The color fades slightly with each shampoo, and the effect lasts for about 15–20 shampoo cycles.
Features: Ammonia-free, gentle and non-damaging, does not bleach or brighten hair, soft and natural color, ideal for those who like short-term color changes or are trying hair dyeing for the first time.
2. Semi-Permanent Hair Dye (Demi-Permanent Dye)
This type of hair dye uses a low concentration of developer (level 3–10), which only slightly opens the hair cuticle and deposits the pigment onto the natural melanin layer.
Because the developer does not contain ammonia, it will not brighten or bleach the hair.
Features: Longer lasting than semi-permanent, natural and soft color, less damaging to hair.
Disadvantage: It cannot significantly brighten the hair color, but it is a very good choice for those who want to cover early gray hair or adjust the color.
3. Permanent Hair Dye
This is the most common and longest-lasting type.
Different levels of developer are required depending on the desired degree of brightening.
Developer Level Explanation:
Level 10 (3%): Used to increase the tone of the same color level, does not brighten, suitable for touch-ups.
Level 20 (6%): Can brighten 1–2 levels, suitable for dyeing gray hair.
Level 30 (9%): Can brighten hair by 3 shades.
Level 40 (12%): Can brighten hair by 8 shades ( Not recommended for home use, high risk).
The higher the developer concentration, the stronger the bleaching power, but also the greater the damage to hair.
Most commercially available boxed hair dyes have a developer concentration of approximately 7% (about level 20), resulting in a relatively mild but less vibrant color.
Tip: I personally usually use level 20 developer when dyeing my roots at home, which brightens the color without damaging the hair.
III. Natural Hair Color Grade: The Foundation for Hair Dyeing Results
All hair colors can be divided into 1–10 levels:
1 is the darkest pure black
10 is the lightest blonde
The most natural effect is achieved when the dyed color differs from your natural hair color by no more than two shades.
For example: My natural hair color is a level 7 warm brown. I can safely dye it darker to level 5 (dark brown) or lighter to level 9 (light brown).
Anything beyond this range will require bleaching first; otherwise, the color won't show up.
IV. Undertone of Hair Dye: The Key to Determining Warm and Cool Tones
Hair dyeing isn't just about "light and dark variations"; the undertone is equally important. The undertone determines the warmth or coolness of the hair color and its harmony with your skin tone.
Common undertones include: Neutral, blue, purple, gold, green, gray-gold, brownish-purple, brownish-red, red, and reddish-purple.
If you naturally have a warm skin tone and your hair appears golden in sunlight, cool tones (such as gray, silver, and blue) may make your complexion look dull.
Conversely, warm tones (golden brown, copper red, and gold) make your complexion look more vibrant and radiant.
For example, I myself am a warm-toned level 7 brown, and dyeing it gold (level 8 red-gold base) looks particularly natural; while a cool platinum gold can make my complexion look grayish and older.
V. Hair Dye Shade Grades: Understanding the Numbers for Precise Color Selection
Just like natural hair color has varying shades, hair dye shades are actually composed of two parts: a "tone level" and a "base color."
Understanding their combination will help you more scientifically create your ideal hair color.
For example, in my recent golden brown hair color formula, I mixed the following shades:
2N Obsidian Satin Black – Deep, pure black, enhancing root layering.
4N Velvet Espresso – Velvet deep brown, adding overall richness.
5N Cashmere Mocha – Soft mocha brown, making the color transition more natural.
6N Golden Hour Linen – Golden brown shine, adding warmth and luster to the hair.
I used a 10-level developer (3% Developer) for overall coloring, creating a smooth transition between the light and dark shades, resulting in a natural yet elegant layered hair color.
This combination is perfect for those wanting a natural, soft, and subtly enhanced look—the 10-level developer maintains the depth of the darker shades, giving the hair a more natural sheen.
If you desire a brighter, more translucent color with a more pronounced golden brown, choose the 20-level developer (6% Developer). This will brighten the hair by about 1-2 shades, making the 6N golden brown tone stand out even more.
This multi-color blend not only creates natural gradations between light and dark shades but also presents subtle variations in light and shadow, better complementing skin tone and hair shine.
VI. Why do hair dyeing results vary?
Key factors affecting hair dyeing results include:
Natural hair color level—the darker the hair, the harder it is to brighten.
Hair health—damaged hair absorbs color quickly but also fades quickly.
Developer concentration—determines the degree of color brightening.
Dyeing time—too short a time won't result in proper coloring, too long a time can lead to color deviation.
Therefore, the same hair dye will produce different results on different people.
VII. Conclusion: Understanding the Principles Leads to More Beautiful Hair Dyeing
Hair dyeing is never a blind "coloring game," but a scientific and artistic experiment.
Understanding the principles of hair color grades, developer concentration, and base color,
will help you avoid pitfalls and minimize hair damage when dyeing your hair at home.
Now, more and more brands are launching ammonia-free, plant-based hair dyes,
which not only provide natural color but also nourish the hair during the dyeing process,
transforming hair dyeing from a "chemical process" into a gentle aesthetic experience.
In short: "A good hair dye not only changes your color but also protects your hair and overall look."
