Setting *color_rgba properties

[<charlie cucaera co uk>]

I've spent the past few hours racking both my brain and the internet for
this one, any help would be appreciated!

I want the user to be able to set the colour of an item using a gtk color
button; how do I pass the selected colour (a gdk color) to the appropriate
*color_rgba property?

I've tried extracting the red, green and blue values from the gdk color,
but I can't seem to convert it to the format specified by the *color_rgba
properties.

Am I right in assuming that 0x3cb3f180 can be broken down into 3c = red, b3
= green, f1 = blue, 80 = alpha? How can I create this from the gdk color
rgb values?

Specifying the colour in the source works fine; but isn't particularly user
friendly...

Thanks,
Charlie Barnes