First thing I replaced on my Rocks were the grips. The standard "factory" grip is fatter than Rosie O'Donnell at a cupcake factory.
As for the color of the walnut, it depends on the type and grade of walnut being used. I wouldn't expect Cimarron/Rock Island to be using high grades of walnut on their grips. Maybe English Walnut (it tends to have a lighter hue than the Black Walnut) in a utility or semi-fancy grade. You are dealing with a pistol that's generally under $600.
Experience is a cruel teacher, gives the exam first and then the lesson.