You can close a small miter gap by rubbing it with a screwdriver shank or any hard, smooth tool. We used the end of a utility knife. That crushes the wood fibers inward to make the gap disappear. Even professional woodworkers sometimes resort to this.
Whether you're banding a tabletop or making a picture frame, make sure the wood color and the grain pattern match at the miters. Selecting matching wood at the lumberyard takes only a few extra seconds and gives you much better-looking miters.