How can I add a alpha channel to to pixbuf remove a color when paint?

["chen.shengqi" <chen shengqi connet com tw>]

Dear sirs:
    How can I add a alpha channel (opacity information) to a pixbuf. so I can remove region with desired 
color when paint is to Drawable. for example, I have a pixbuf is a blue rectangle with a yellow text on it. I 
want to remove the blue color from it when paint it to another pixbuf,just like only draw the text.
    thanks.