If you wish to accent the rim cap, it should match the fingerboard. In the case you don’t plan to accent it, it could match the neck. The rim cap covers the laminations that would otherwise be visible on the backside of the rim. The cap is usually made from four or five thin pieces of wood butted together. I like to cut the ends of mine so that they slant into each other. In the case of ebony, the butt joint often becomes nearly invisible. A power sander is handy to have around to help with fitting the joints together. It also helps to cut the cap pieces a bit wider than the rim. This allows for a little play when gluing them in place. Wait until all the others are glued in place and set, then adjust the edges of the final piece to make a nice clean joint on both ends. After the glue has set, trim them all nearly flush with the outside and inside of the rim with a flush cut router bit or use the rasp and file. The rim cap may be put on either before or after the lathe work.