There is no invisible colour, as such - you have to save a GIF and set its transparent colour in your paint app (PNG is the same, or with an alpha channel - different levels of transparency, mmm).
ah...i guess i'm stuck in the old days being hooked to old games like doom and duke 3d where the transparent color has to be a certain color (lightblue in doom, purple in duke3d)...heh
Yep, if your using Photoshop simply right-click on the background and select Layer -> Delete -> Layer and it'll make the background transparent (provided you save in a format that supports transparency).
Mike
"Now I guess we're... 'Path-E-Tech Management'" -Dilbert