[Image-SIG] Put alpha on a color

[Angelo]

Hi,

I'm new with PIL, and I want to know how to put alpha channel on a color
(not simply a band) . 

putalpha(band) works only with one band at the same time, it is that? .
I would like to make a trick of the style ==> putalpha((108, 128, 199)),
that is it possible (?), and how to make? .

Thanks in advance .

monsitt