Alpha: 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
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).