Unethical? Don't know why it would be unethical. It would probably be rude, unnecessary and in poor taste.

Here is an uncomfortable moment.
Rashida Jones is the best!

Depends on the context, between friends in a light hearted manner why not but if in a disparaging engagement no.

It’s no different to pointing out a physical feature of anyone and accentuating it. It’s always about context.

A similar question of etiquette:

When we were sailing off the coast of CA, one of my white friends got out the sun block and started passing it around. It got to me, and I put some on, and offered it to my Africa American friend. He declined and passed it on.

Did I handle that correctly?

There is no one tone of black skin, just as there is no single skin tone for any ethnicity and thus yes a black person can get a tan and might well enjoy being complimented if the intent is complimentary rather than mockery.
My school friend Courtney came from St. Vincent and was very dark skinned but his older sister who was his guardian had a different father who was white and thus had far less melanin but was still obviously west Indian, during the summer she would sun bathe and her skin would darken just like anyone else, and never took it as an insult to be complimented on her noticeable suntan (she was a very attractive woman and was admired by most of Courtney's friends so got plenty of compliments anyway, well it was the seventies).

I'm with some of the others here. Depends on how well you know them. Most all of my friends are just as crazy as I am and nothing we say to each other is meant to be offensive and we all seem to know boundaries. I would never say that to someone that I didn't consider a friend.

Is it racist? No, it is an expression of a mental disability, or as they say in Boston... Wicked Retarded.
Of course racism is in itself an unwillingness or inability to think and comprehend.
But I guess it would be up there with complimenting a white person on their paleness.

My mixed granddaughter talks about being tanned in the summer. Her brother also refers to his tan lines

Perhaps it is different because we live in Florida where a person will get darker no matter their original skin tone nearly every black person I know has tan lines and calls them tan lines

Doesn't exist for them

I think the intent is not the issue. Perhaps a way to pose the question in a manner that is not so off putting. I really never, ever heard a black person use the term tan unless mentioning or discussing with non blacks. I am not the gatekeeper of "blackness". So, my experience, tan is not in my vocabulary at all. For example, the term "Ashy" will probably prompt a Google search if you are not black. I darkened significantly during my military training as did everyone else, but I would have never applied the term "tanning" to myself (and it was temporary, lasted a some months?????)

It is just finding a common term I would submit. I kid about working on my tan, after a run when I relax in the sun, but "tanning" is not an objective. I hope that helps a bit. To your question, it is not unethical or racist, it is just that the language is awkward and if the person is a stranger it can be a bit strange. I have been in 5 foreign countries, so I understand the curiosity. Just differences, not bad or good.

I am a white person and I don't think I'm at all racist. I might comment on a black person's depth of black color simply as an observation but not to the person as I'm sure they are already aware. I might also comment on a white person's white (albino?) but also not to the person. Shouldn't we get to a point where we no longer need to consider race in conversation in order to put racism behind us? At this point in time I would not comment on a tan to a black person due to the a potential misconception. I don't believe we're there yet. Interesting question though. I'm interested in your opinion though.

I compliment all of my friends appropriately...everyone wants to look good...my black friends ask me all the time how I stay so pale even when I go to the beach...I use SPF 70...are they white shaming me? Nope...they know I don't want wrinkles...

It depends on how well you know them... and I agree with the answer below mine.

I think if you know someone personally, and you know they are working to enhance their natural skin color via tanning, and you compliment them, it is okay. Other than that situation, I don't see how it would be appropriate to say something.