思路: 将不同的文字，设置为不同的NSAttributedString (其中包含颜色，font等属性)
let kindLb = UILabel(frame:CGRect(x: screenWidth/2 - 100, y: remainNumTitleLb.frame.maxY + 10, width: 100 , height: 17))
let string = "steve_Moriya"
let aa = (string as NSString).range(of: "_")
let bb = (string as NSString).substring(to: aa.location)
let cc = (string as NSString).substring(from: aa.location + 1)
let kindLbStr1 = NSAttributedString(string: bb, attributes: [NSAttributedStringKey.foregroundColor : UIColor.white, NSAttributedStringKey.font : UIFont.systemFont(ofSize: 20)])
let kindLbStr2 = NSAttributedString(string: cc, attributes: [NSAttributedStringKey.foregroundColor : ThemePurpleColor, NSAttributedStringKey.font : UIFont.systemFont(ofSize: 10)])
let kindLbStr = NSMutableAttributedString()
kindLbStr.append(kindLbStr1)
kindLbStr.append(kindLbStr2)
//UILabel和UIButton直接使用 KindLbStr
kindLb.attributedText = kindLbStr
UIButton的设置方法有一点不同
let rigistButton = UIButton.init(type: UIButtonType.custom)
rigistButton.frame = CGRect(x: 40, y: loginButton.frame.maxY + 20, width: 160, height: 20)
rigistButton.setAttributedTitle(kindLbStr, for: UIControlState.normal)
