In 1959, GM painted the rims in body color, unless the car came equipped from the factory with full wheel covers, and in that case, they were semi gloss black.
I suggest that you use paint, rather than powder coat, since powder coat can chip, and once water gets under the powder coat, the metal will rust severely, since it can't get out. I have seen this first hand on parts of my tractor that were powder coated, and others were painted. The painted surfaces are still rust free, although after 10 years of being out in the elements, it has dulled. The powder coat still looks shiny, but it has pealed on the edges and the metal has rusted. One attachment that was fully powder coated, rusted so badly, that it was not usable and junked. There are two types of powder coat, one is extremely flexible and doesn't chip, and the other (most common) is like a rigid plastic, that chips and cracks. I see powder coat as an expensive alternative that isn't as good as the original process used when the car was new. Unlike paint, once applied, it is difficult to remove totally, and unlike paint, it can't be repaired. How difficult is it going to be to find replacement original wheels if you were to need them? Is powder coat really worth the risk, and it cost more than paint.