Objective-C highlight UIButton when selected / unselected

I have a UIButton inside a UIView, when the UIButton is selected I would like have a background colour of dark gray...is this possible?

UIView *toolbar = [[UIView alloc]initWithFrame:CGRectMake(10, 50, 40, 160)];
    [toolbar setBackgroundColor:[UIColor colorWithWhite: 0.70 alpha:0.5]];

    toolbar.layer.cornerRadius = 5;

    UIButton *penTool = [UIButton buttonWithType:UIButtonTypeCustom];
    penTool.frame = CGRectMake(5, 0, 30, 30);
    [penTool setImage:[UIImage imageNamed:@"pen-but" inBundle:currentBundle compatibleWithTraitCollection:nil] forState:UIControlStateNormal];
    [penTool addTarget:self action:@selector(drawButtonTapped:) forControlEvents:UIControlEventTouchUpInside];
    penTool.autoresizingMask = UIViewAutoresizingNone;
    penTool.exclusiveTouch = YES;
    penTool.tag = 1;

    [toolbar addSubview:penTool];