A swiftmend spell on an actionbar has a somewhat complex set of conditions for when it is darkened or lightened. If there are no heal over time (HoT) spells on the current target it is always darkened. Otherwise, if there is at least one charge of swiftmend available it is completely lightened but if there are zero charges then it is partially lightened (the part that has already been swept by the sweeping animation).
I would like to keep all of this except that first condition: I do not want it to darken if there are no HoTs on the current target (since I do click casting and don't target those I heal).
This was my first attempt but it does a partial darkening even when I have a swiftmend charge available: