Your quilt is very pretty and fresh looking. If you know who is getting the quilt, you can pick out a border color. I think a pink solid would be a great choice, unless the quilt is for a man or boy. In that case, I'd use a royal blue or a deep green.
It also depends on what size you'd like the quilt to be. Some styles of quilts look great without a border, and yours is in this category as well!