on OSX I have an NSButton with a pretty dark image and unfortunately it is not possible to change the color using the attributes inspector. See picture the big black button,
This is how I get it done in Swift 4
@IBOutlet weak var myButton: NSButton!
// create the attributed string
let myString = "My Button Title"
let myAttribute = [ NSAttributedStringKey.foregroundColor: NSColor.blue ]
let myAttrString = NSAttributedString(string: myString, attributes: myAttribute)
// assign it to the button
myButton.attributedTitle = myAttrString