uglyducking has his point.
The silver colour (or any colour) on rim is just somekind of paint over the alloy material.
So, when you think carefully, there are 3 layers:
1) tire
2) paint
3) alloy material
We all see colour diseemble from alloy wheels, so will the colour around the rim edge.
So, if paint is separated and so air will slowly leak between layer 2) and 3).
If that happens, a clean is required to make sure layer 1) and 3) is attached again