As pointed above one way to modify an accessoryView is by adding your own UIImageView. However, in fact you can supply any object deriving from UIView. I would then recommend using a UILabel with an icon font (e.g. icomoon) instead of UIImageView. UILabel and an icon font allow for flexibility in both image, color and size.
let font = UIFont(name: "icomoon", size: 16.0)
let icon = "\u{e60f}"
let lbl = UILabel(frame: CGRectMake(0, 0, 20, 20))
lbl.font = font
lbl.text = icon
cell.accessoryView = lbl