Most of the grand jurists have said in regards to having tattoos: If tattoos are mere color or they are done under the skin and there is no substance on the skin to prevent water from reaching the skin, wudhu, ghusl and prayer are in order.[i] If there are substances on the skin that form obstruction and prevent water from reaching the skin, wudhu, ghusl and prayer would be invalid.[ii] In the latter case, the obstruction should first be removed and then one should take action for ghusl.
In case, the tattoos contain any substances or ingredients which prevent water from reaching the skin, it is necessary to remove them before wudhu or ghusl.[iii] In case, the tattoos do not contain any substances that would form obstruction, there is no problem. Therefore, hajj acts like Tawaf (circumambulation), and prayer of Tawaf which need ritual purity (wudhu or ghusl) are in order in such a case.
For further information, vide: Index: Validity of Wudhu with Tattoos, question No. 1865 (site: 1860).