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,
NSColor color = NSColor.White;
NSMutableAttributedString colorTitle = new NSMutableAttributedString (cb.Cell.Title);
NSRange titleRange = new NSRange (0, (nint)cb.Cell.Title.Length);
colorTitle.AddAttribute (NSStringAttributeKey.ForegroundColor, color, titleRange);
cb.Cell.AttributedTitle = colorTitle;