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,
A really simple, reusable solution without subclassing NSButton:
[self setButton:self.myButton fontColor:[NSColor whiteColor]] ;
-(void) setButton:(NSButton *)button fontColor:(NSColor *)color {
NSMutableAttributedString *colorTitle = [[NSMutableAttributedString alloc] initWithAttributedString:[button attributedTitle]];
[colorTitle addAttribute:NSForegroundColorAttributeName value:color range:NSMakeRange(0, button.attributedTitle.length)];
[button setAttributedTitle:colorTitle];
}