I think the rails s/b the same look as your floors for the best results IMO. White balusters to compliment the look would be stunning IMO.

Try L.J. Smith for the best selection of products to use. Cumaru can probably be custom made but would be very expensive. LJ Smith has several choices in various woods and various prices. We put in antique heart pine floors and to have custom rails made from the same materials was going to be outrageously expensive. I selected several good hard woods that stain well and got some samples of maple and poplar. I then mixed my own stain, trial and error in small amounts recording the ratio of parts and stained the samples until I got a good match. In the end, I found poplar was easiest to match in my case.

I then handed everything over to the painter and he stained and finished the rails and they match perfectly! The final finish is Waterlox. Waterlox is a tung oil based varnish product that is very durable and very suitable for a handrail. It will protect against oils, dirt etc and cleans up quite well.

I know it seems odd that i mixed my own stain but it is not that hard and it was the only way I could be sure I was happy with the color. In short, insted of telling the painter "how about trying a little more of this color and see" I just looked at it and used my own judgement (and time and patience). BTW, I recommend oil based gel stains. This was the first time I had used the gel stains and I found them to be quite nice to use and very forgiving (even for my painter). Also, I had read a lot about preconditioning wood before staining. I found maple and poplar to stain very evenly and well and took on a much more rich look without using a wood conditiner.

Sorry for the long winded response but I hope it helps. Having been there and done that, I know something like a hand rail seems like such a little thing but it makes such a powerful statement when done well.