Is it permissible to open a beauty salon for women?

A woman adorning herself for her husband is religiously مطلوب (required/desirable) for the continuation of affection and love. If the wife is not skilled in this, it is permissible for her to seek the help of another woman to perform this work.

Opening beauty salons for women is not forbidden, provided that the work is restricted to adorning only those women who are beautifying themselves for their husbands, and that their adornment is not displayed to non-mahram (definition) men, and that it does not become a source of fitna (temptation) for people, and that the work is free from forbidden acts such as plucking eyebrows (namṣ) and hair extensions (waṣl al-shaʿr) and looking at awrahs (private parts).

However, if these salons are frequented by both the righteous and the immoral, and by those who wear hijab (definition) and those who do not, or if they involve anything forbidden, then it is not permissible to open them or work in them, because that would constitute cooperation in sin.

If you know yourself to be capable of controlling who comes to the salon to ensure that no disobedience is facilitated, then there is no harm. Otherwise, you must stay away from this field and seek another permissible one.