Trigger and action slots look the same. In addition, some gates have trigger slots on the right of action slots. It might be useful to display a tooltip or textbox above the trigger and action source slots to identify them as such. (or alternatively just use the existing help ledger for this).

It should be possible to try this out using a resource pack, thanks to the json gui stuffs (as currently slots probably use the same icon for a blank slot). If this isn't possible currently then that functionality should be added.