I dont think anyone has ever come down on a definite decision - I have heard people recommend pearlising medium and also interference paint...but there are many different types of pearlising mediums different finishes, different viscosity so you need to make sure you get the right one, I think the thinner the better. However, this doesnt restore the rainbowish sheen just a pearly coat. You also have to be careful not to apply too much as it can leave a milky texture or even a streaky finish.
Personally I would advise against it if its only a mild issue as its not worth the potential risk both in damage and in the fact the finish will not be like on the original pony. But if the pony is baity restore and you want to give it a try then maybe its worth a try out to see if you can get the balance.