Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is known that applying a material which is considered just a color on the skin, such as henna, does not nullify/prevent wudu. However, other materials that make a layer on the skin that prevents water from reaching it are to be removed for wudu.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
If it is simply a color without any layer preventing water from reaching the skin, then there is no need to worry. If, however, there is a coating, it should be removed for wudu.
Washing hands–nails included-with water is an essential part of wudu, which means that wudu is not valid without it; washing is incomplete unless water comes into direct contact with the skin/finger.
Henna, however, is different from that of nail-polish since it has no visible layer but only color. So it does not affect the validity of wudu.
What is stated above is the generally accepted view of all the schools of jurisprudence? Exceptions to this rule are made only when use of water becomes harmful because of a health condition or sickness.
Allah Almighty knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.