White ink like most other tattoo inks are transparent. Unless you use it along others colours or to do certain details and effect it won’t show as pure opaque white.

When white tattoos are healed they look really subtle depending on the person. Sometimes they look like faded scars, etc. You can look it up, that’s roughly how they look.

As for UV tattoos, I think the ink is not really regulated which mean little to no information on risk and its properties in general. Some say it causes cancer, while most have reported skin troubles.

We don’t do it where I work.
If you want something UV, stick with wearables like clothing, accessories, or even hey body piercings!