It seems – and Allaah knows best –that what is put on the hair or used to make the hair stick together, such as henna and the like, does not affect wudoo’, rather it is sufficient to wipe over it.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
If a woman plasters her hair with henna or something similar, she may wipe over it, there is no need remove the substance in question, because it was proven that the Prophet (peace and blessings of Allaah be upon him) used to apply something to make his hair stick together when in ihraam.
See al-Sharh al-Mumti’ by Shaykh Ibn ‘Uthaymeen, 1/196
What is put on the head and makes the hair stick together becomes part of it, which indicates that there is some lenience with regard to purification of the head.
And Allaah knows best.