code: public com.google.protobuf.FloatValue getAlpha() {
      return alpha_ == null ? com.google.protobuf.FloatValue.getDefaultInstance() : alpha_;
    }
docs: <pre>
  The fraction of this color that should be applied to the pixel. That is,
  the final pixel color is defined by the equation:
  pixel color = alpha * (this color) + (1.0 - alpha) * (background color)
  This means that a value of 1.0 corresponds to a solid color, whereas
  a value of 0.0 corresponds to a completely transparent color. This
  uses a wrapper message rather than a simple float scalar so that it is
  possible to distinguish between a default value and the value being unset.
  If omitted, this color object is to be rendered as a solid color
  (as if the alpha value had been explicitly given with a value of 1.0).
  </pre>
  
  <code>.google.protobuf.FloatValue alpha = 4;</code>