Is skin Tanning allowed during 9 days?
Technically it might be permitted, but it is not a proper thing to do during the nine days.
Shemuos Moshe 550-1, Mah Tovu 1 pg. 68 in the name of R’ S. Diblisky zt”l and Ybchl”ch R’ A. Nebenzahl shlit”a who both said (regarding tanning on a fast day) that although it is technically permitted it is not befitting with the atmosphere of the time. If this is the case regarding fast days, it should surely apply to the nine days.
As a side point regarding tanning on Tisha Ba’v itself, R’ Moshe zt”l said that it is forbidden. See Shmaytisa D’moshe 555-2.