So, there's a whole movement of people tanning their perennial area…
And penises are no exception.
The thought here is that if you can somehow tan this, the penis and testicles, you'll increase your androgen. In reality, this is probably a very dangerous practice. Since these areas are not typically exposed to light, they can easily burn like any other area of skin.
In addition, these areas are much more sensitive and the risk of skin cancer like tanning. Any other area of the body is still there.
So, the verdict is probably a bad idea to put sun tanning lotion on your penis because it's not really designed to be an absorber of sunlight.
If you are looking to highlight your private area, non-surgically enhancing the penis is perhaps the safest option.