code: public boolean shouldYieldFocus(JComponent input) {
          boolean isValidInput = verify(input);
          if (isValidInput) {
              input.setBackground(background);
          } else {
              input.setBackground(warningBackground);
          }
          return isValidInput;
      }
docs: Verifies the input with the side effect that the background color of {@code input} to {@code background} if
  returns {@code true}, or {@code warningBackground} otherwise
  
  @param input component
  @return if input is valid