I think negative alpha value should be possible.

When these alpha channels are used they will cancel out alpha channels of other icons. So I can make a "darkness" icon which is rgb(0,0,0,128) and a "light" icon which is rgb(125,125,25,-128) (yellowish).

The result is that when the latter icon is displayed over the former it will cancel out the blackness of the icon, whereas if the latter icon is displayed over anything else it will display a yellow haze like it would if it had rgb(125,125,25,128).

I hope you understand this. :\

-- Data