Johnny -
I can tell you the approach that I would take, and, when BowDoc chimes in, he can tell you the approach that you probably SHOULD take.
If I planned to hunt with the bow, instead of trying to show it off, I would not mess with trying to get a plug that matched the grain of the riser. Instead, I would take a more expedient approach.
I would find a piece of wood that was ~1/8" (or so) shorter than the depth of the hole and that fit the hole snugly. This piece of wood could me a piece of a dowel or something whittled to size. I would glue this piece of wood in the hole leaving ~1/16" of space on the ends. I might use superglue or I might use 5-minute epoxy, whatever seemed best based on how snugly the piece of wood fit the hole (epoxy would fill gaps better). After the plug was securely in place, I would mix up epoxy to fill the last 1/16" on each side. I have mixed sawdust and (Testor's Model) paint with epoxy whichever matched the color of the wood better. The epoxy + sawdust made for a coarser surface, so I would plan to sand it down after it had set. BowDoc has said that he sometimes uses powdered artist paint pigments to color epoxy and superglue, both. After the final fill is fully set, I would sand to create a better surface and put on a little finish to seal the deal.
Okay, now BowDoc can chime in and tell you what you should really do.
Ray