How do I change the color of two UIButtons when I press them?

@IBOutlet var firstButton: UIButton!
@IBOutlet var secondButton: UIButton!

// computed var to access your buttons
private var buttons: [UIButton] {
    return [firstButton, secondButton]
}

override func viewDidLoad() {
    super.viewDidLoad()

    // one setup to configure each button for selected or not selected
    buttons.forEach { button in
        button.setTitleColor(.darkGray,
                              for: .selected)
        button.setTitleColor(.lightGray,
                              for: .normal)
    }

    firstButton.isSelected = true
}

@IBAction func buttonAction(_ sender: UIButton) {

    UIView.animate(withDuration: 1.0) {
        // flip buttons
        self.buttons.forEach { button in
            button.isSelected.toggle()
        }

        if self.firstButton.isSelected {
            // tweak your constraints for firstButton.isSelected == true
            // tweak your constraints for secondButton.isSelected == false
        } else {
            // tweak your constraints for firstButton.isSelected == false
            // tweak your constraints for secondButton.isSelected == true
        }

    }
}