swift 5 version:
By using default inbuilt color:
button.setTitleColor(UIColor.green, for: .normal)
OR
You can use your custom color by using RGB method:
button.setTitleColor(UIColor(displayP3Red: 0.0/255.0, green: 180.0/255.0, blue: 2.0/255.0, alpha: 1.0), for: .normal)