Honestly, it's really personal preference. There's several viable options.
You could start from the black anodized or weathered finish as a base and sand it to taste (scotch brite pads work great). This is probably the simplest method, and can be used as a base for the next one:
You could take off most of the weathered/black finish or start from the dual tone finish and use spray paint (flat black or aluminum black are the most common), then when it dries sand it a bit. An rag soaked in acetone is nice to break up the paint without really taking it off completely like sanding would; it also sort of blends the areas together.
After using paint you may wish to top it off with an enamel clear coat. This will prevent it from wearing any further... should you not want it too. If you don't clear coat, it will naturally weather more over time with use.
Another style that I've seen done is via chemical weathering. I picked up an empty hilt that was done with ferric chloride and then aluminum black paint. That made it a mix of brown and grey.