I think there are two ways:
1. the pearl is allready filled and you want it a bit darker/browner/vintage you could add a drop iodine from behind into the hole, which should coulor the pearl inside a bit.
2. You scratch everything out und mix iodine and Revell up to you desired colour and fill it into the pearl.
Both ways should give you a new coulor, way number 2 should give you the exact coulor you want, way number 1 is try-and-error, but ether ways you can´t destroy anything, so you could try way 1 and if it fails try way 2...
BTW: iodine also works perfect on hands ;-)