It's essential to remember that the actual results may vary depending on factors such as hair type, texture, porosity, and the presence of artificial color or previous chemical treatments. Here's a closer look at the standards you've mentioned:

1. Bleaching powder + 6% hydrogen peroxide milk – bleaching 2~3 degrees: This combination is suitable for individuals seeking a mild to moderate lightening effect, typically aiming to lift their hair color by 2 to 3 levels on the hair color chart. For example, someone with natural medium brown hair (level 5) could expect to achieve a light brown or dark blonde result (levels 7 to 6). This lower-volume developer is generally gentler on the hair and may be preferred for those with delicate, fine, or previously chemically treated hair.

2. Bleaching powder + 9% hydrogen peroxide milk – bleaching 3~4 degrees: This pairing is intended for more substantial lightening, targeting a lift of 3 to 4 levels on the hair color chart. It may be suitable for individuals with darker hair shades looking to reach a lighter brown or blonde tone, or for those with lighter hair seeking to remove stubborn artificial color or achieve a more dramatic transformation. The higher concentration of hydrogen peroxide provides faster and more effective lightening but can be more damaging to the hair, especially if left on for extended periods or if the hair is already compromised.

3. Bleaching powder + 12% hydrogen peroxide milk – bleaching 4~5 degrees: This combination is recommended for the most intensive lightening, typically aiming to lift hair color by 4 to 5 levels. It is commonly used for very dark hair shades transitioning to light blonde or for heavily pigmented or resistant hair that requires substantial lightening. The highest-volume developer offers the fastest and most potent lightening power but is also the harshest on the hair, increasing the risk of damage, breakage, and dryness. Professional supervision or experience is highly recommended when using 12% hydrogen peroxide milk for bleaching.

Keep in mind that these standards serve as general guidelines, and the actual lightening results may vary depending on individual hair characteristics and the specific bleaching process. It's crucial to perform a strand test, closely monitor the bleaching process, and adjust the processing time accordingly to avoid overprocessing and minimize damage. Additionally, following proper aftercare and using nourishing hair treatments can help maintain the health and integrity of the hair post-bleaching.