The opacity of the color from 0.0 for completely translucent to 1.0 for opaque
The intensity of the blue channel from 0.0 to 1.0 inclusive
The intensity of the green channel from 0.0 to 1.0 inclusive
The intensity of the red channel from 0.0 to 1.0 inclusive
A #GdkRGBA is used to represent a (possibly translucent) color, in a way that is compatible with cairo’s notion of color.