Is it permissible to remove lower garments on the beach for the purpose of tanning, given that the majority of girls there do so and there is no lustful gaze?

A woman being naked and participating in nudity is a great evil, and the husband must forbid his wife from doing so.

The claim that naked people do not look at each other with lust is a false assertion, and looking at another person's 'awrah (private parts) is absolutely forbidden, whether with lust or without it.

A woman's 'awrah in front of other women is from the navel to the knee, and in front of men, it is her entire body except for the face and hands, according to some scholars.

It is not permissible to expose the 'awrah except out of necessity or urgent need. There is no necessity or urgent need for the aforementioned dyeing to be done naked. If it must be done, then it should be away from sight; otherwise, it should be abandoned.

Doing so is definitively forbidden, and repentance for what has been mentioned is obligatory.