code: private void setColor(String key, String color) throws HelloSignException {
          if (!validateColor(color)) {
              throw new HelloSignException("Invalid color: " + color);
          }
          set(key, color);
      }
docs: Helper method to validate and set a color.
  
  @param key String key
  @param color String color hex code
  @throws HelloSignException thrown if the color string is an invalid hex
  string