One of the easiest ways to accessorize a shelf or bookshelves, is to add art, framed or not. I collect postcards from museum of prints from my favorite artists. Other options could be note cards, greeting cards, pages from calenders, and Etsy; Etsy has sooooo many fantastic small options. Framed art is definitely more structured, unframed art is more casual, both are fabulous.
Another great way to add "color" and visual impact is colored paper. Paper should obviously be framed, with or without a mat.
When using a mat, anything goes. Jo-Anns offers mat remnants for super cheap, in an amazing assortment of color.