Comment #1 on issue 101 by
00a...@gmail.com: Add alpha to mixer
http://code.google.com/p/gpick/issues/detail?id=101
Are you referring to displaying the current color over a white background
with the level of opacity you choose?
Currently, you can achieve this using the 'Color mixer' secondary panel,
which can be activated by opening the View menu, choosing 'Secondary view'
and then choosing 'color mixer'. By dragging your desired 'background
colors' onto one of the left set of colors, and dropping the color you want
to mix onto the large central color area, you can see the result on the
right. Opacity is adjustable using the slider at the bottom.