In "The One with Ross's Tan" Ross gets too much spray-on tan. What was actually done to the actor to simulate the tan. (I assume he did not actually do the thing which happened to the character.)
Most probably makeup, for how to achieve it refer, Wikihow :
There are plenty of makeup tricks to give you the gorgeous tan you've been looking for, many of which are easy to do. Using makeup is the safest, but also most temporary, way to darken your skin. One great method is using bronzers and shimmer powders. You'll need a few simple products to create your new tan look, all of which can be found online or at any makeup department or convince store.