The most significant "clue" by which other people will judge the shade of your skin, is the shade of the skin of people around you.
If you're standing in a group where most of the people have skin significantly lighter than yours, your skin will be perceived as relatively dark. If you're in a group where most of the people have skin significantly darker than yours, your skin will be perceived as relatively light.
It's much like height - a 6 foot (1.83m) tall man standing in a group of people who are shorter than 5'6" (1.67m), will appear tall. If the same fellow is surrounded by NBA players (who tend to be closer to 7' (2.13m) tall, than to 6'), he'll be perceived as short.
You'll doubtless be told that wearing dark colors will make your skin look lighter. There's a kernel of truth to this, but only a kernel. If there were no people around you, the darker fabric might cause an onlooker to be biased slightly toward judging your skin tone as being comparatively light. But if there are any people around you, the human mind will almost always default to measuring you (your height, weight, skin tone, posture, etc.) with reference to those other people. Human are the preferred frame of reference, if you will, when it comes to judging humans attributes.
Frankly, if your skin tans darkly, and you want to appear as light-skinned as possible, you'll be far, far better off simply not acquiring a sun tan, than you would be adopting a clothing-related approach. Wear long sleeves, to keep your arms from acquiring a dark tan. Wearing sun screen might also be a good idea (which might also have some health benefits).
(I'm intentionally ignoring the likelihood that by attempting to appear as light-skinned as possible, you're actually perpetuating existing racist attitudes. That may be a valid argument, but I'm also convinced it's not one appropriate for inclusion in the Menswear Advice forum.)