def with_alpha(self, alpha):
return Color(self.__rgb, '<STR_LIT>', alpha, self.__wref)<EOL>
Create a new instance based on this one with a new alpha value.

        Parameters:
          :alpha:
            The transparency of the new color [0...1].

        Returns:
          A grapefruit.Color instance.

        >>> Color.from_rgb(1.0, 0.5, 0.0, 1.0).with_alpha(0.5)
        Color(1.0, 0.5, 0.0, 0.5)