how do i access a ui button in the viewcontroller class from a function parameter?

Your code is wrong, specifically the argument types for changeIconColor. For example this should work:

class ViewController: UIViewController
{
    @IBOutlet weak var btnMaxAloud: UIButton!

    override func viewDidLoad()
    {
        btnMaxAloud.tintColor = UIColor.white
        changeIconColor(btnMaxAloud, UIColor.red)
    }

    func changeIconColor(_ button: UIButton, _ color: UIColor)
    {
        button.tintColor = color
    }
}

The function changeIconColor() now takes a UIButton and UIColor as argument.