Hi Stan,
Please don't take what I said as definate.... I don't really have a good eye for these things! I've been known to wear purple trousers, a green shirt with an orange jacket.......... :p
Walnut is about as dark as Ipe, so it probably wouldn't make a difference really. And Ipe is heavier, so it would give some weight to the riser. Not much but some extra....
Big please don't take what I said as a negative remark, I'e got a thing for walnut at the moment.
Please post some pics of it when it's done, and remember, paper thin fades.......