Vintage jewelry, by nature, is usually quite old and will often have minor flaws and blemishes. Rhinestone vintage jewelry, in particular, is prone to various stages of wear, since the foil backing of a rhinestone can sometimes be damaged by moisture, or can easily get scratched if the stones are ever replaced.
Some rhinestone pieces are very valuable and worth the effort and cost of the restoration. There are several places that sell vintage rhinestones. One company that I have used and recommend is Matthew Ribarich's site called MrStones.
But what do you do when you have a vintage rhinestone piece that has badly damaged foil, you have searched high and low, and you still can't find a stone which will blend in well with the others?
Rhinestone foil backings such as those used by Swarovski and other rhinestone manufacturers involves a process similar to that used for the silvering of mirrors. The original process involves some tricky chemical processes. Actual silver is applied to the back of a mirror and then a paint-like coating is applied to protect the silver from oxidation and darkening. Re-silvering a mirror is a process often done and the end result is usually very satisfactory.
To duplicate the process with something as small as a rhinestone would take a miniature set up of the same process and would be quite specialized. You could contact a glass seller who resilvers mirros and ask them if they would undertake the job of resilvering your rhinestones. Don't be surprised if you are greeted with widened eyes and a dropped jaw at your request, since you may be the first person to ask them about this.
A curator at a nearby museum might be able to give you some names of those craftsmen that they use for preservation or restoration. These craftsmen who may know of some other techniques that will assist you. Obviously, this step would only be undertaken as a last resort and if the piece is particularly valuable and suitable for the expense of this type of repair.
If you have a piece which you adore which has damaged stones but it doesn't warrant the expensive of a professional repair, there are vintage rhinestone re-foil kits available. I have seen them on ebay, but haven't tried the product, so I can't guarantee that you will like the results.
Quite honestly, I would be surprised if there is a suitable paint easily applied which will blend well enough with the original silvering or golding. If you do decide to use this process, be sure to remove all of the old silver or gold coating first and then tackle the painting job.
If you do purchase this re-foil product and use it, please publish your results in the comments. I'd be interested to see how it works.