Yeah, you have to download the DISBTN. The BTN path should look like this:
ReplaceableTextures\CommandButtons\BTN<IconName>.blp
The DISBTN path should look like this:
ReplaceableTextures\CommandButtonsDisabled\DISBTN<IconName>.blp
Replace <IconName> with the name of the icon, ofc. Ex:
ReplaceableTextures\CommandButtons\BTNFrenzy.blp
ReplaceableTextures\CommandButtonsDisabled\DISBTNFrenzy.blp
That way, Wc3 will know where to find the disabled icon. Otherwise it shows up as green.
P.S. Could you delete that one trigger 3 posts before this one? ("There's no GUI function line") It is stretching my page, haha.