my suggestion is take the rim to an "automotive crash repair" shop, they will have colour swatches for all models of cars, you will probably find a match in the Bumber bar section of the colour chips.
If no luck, find a Big crash repair shop that uses Dupont Performance Coatings & ask them if the have a Chromovision unit, it's a special camera that takes a picture of the colour & then when hooked up to the computer it searches a data base of hundreds of thousands of colours & comes up with either an exact match or something close enough that they can swing to make exact.