You can set ''Disabled'' (darker) icon to your spell. It is very easy.
Spell will have darker icon, but still castable. Of course, this works for passive icons.
For example, we will take Boots of Quel'thalas.
Paths are:
ReplaceableTextures\CommandButtons\BTNBoots.blp (Normal)
ReplaceableTextures\CommandButtonsDisabled\DISBTNBoots.blp (Darker)
Just paste ''Dark Icon'' path. Same as you use hidden icons.