If you have a tan, your skin has become darker than usual because you have been in the sun.
suntan
She is tall and blonde, with a permanent tan.
她个子很高,一头金发,皮肤黝黑。
tans , tanning , tanned
If a part of your body tans or if you tan it, your skin becomes darker than usual because you spend a lot of time in the sun.
I have very pale skin that never tans.
我肤色很白,永远晒不黑。