Either because you wrote the wrong path or you the icon is not does not have the BTN. Your icon must be named this way:
Command Icon (BTN): ReplaceableTextures\CommandButtoms\BTNMyIcon.blp
Disabled Command Icon (DISBTN):
ReplaceableTextures\DisabledCommandButtoms\DISBTNMyIcon.blp
Passive Icon (PASBTN):
ReplaceableTextures\PassiveButtoms\PASBTNMyIcon.blp
If your icon is still green, then it's because it's because the name is not correct. Your Icon name must have BTN at start. Example: BTNMyIcon . For the others icons, disabled and passive, it's the same things. DISBTNMyIcon, PASBTNMyIcon.
-Rui