[Gimp-developer] What is the purpose of the checkerboard pattern in the color history buttons?

[Ofnuts <ofnuts laposte net>]

In the color history of the FG/BG color selector dialog, the buttons may 
appear split in two along a diagonal with the color on top left and a 
checkerboard pattern on the bottom right. This happens when the color is 
obtained using the color picker button in the same dialog. However I 
can't figure out what this display is indicating. The checkerboard 
pattern would normally hint at some alpha/transparency value, but the 
color displayed has no alpha channel?