Rounded rect button's color can be changed to any color with the background color property in IB as well as through coding, but can not change its color to transparent color easily. For that, you better use a custom button with either rounded image or set layer as demonstrated in the example and don't forget to add Quartz Core framework in you project as well as to import it in your class.
UIBUtton *myBtn = [UIButton buttonWithType:UIbuttonTypeCustom];
myBtn.frame = frame;
myBtn.clipsToBounds = YES;
myBtn.layer.cornerRadius = 5.0f;
[self.view addSubView:myBtn];