If you don't have trim tile already, I would suggest installing a 1" or 2" band of tile on face of the sunshelf, about 2" down from the rounded edge. You should be able to find a tile that matches your current perimeter tile. All of the existing plaster on the sunshelf should be removed prior to re-plastering. The tiled edge will give the new plaster a good place to stop at. It will also separate the two different plaster surfaces so that noticeable differences in color are minimized. It may sound odd, but consider a different color plaster for the sunshelf only. Maybe tan or buff to look like sand. If you go with a completely different look, you won't have to worry about matching plaster color.