For the life of me I just can't figure out why the call to GUIAnnoucements isn't working. I have it enabled in the config file and through the use of plentify debugging messages I've beenable to confirm that the Call to CreateAnnoucement is being reached (with the values I would expect)... 

if (config.NotifyUsingGUI)
{
  GUIAnnouncements?.Call("CreateAnnouncement", notification, config.BannerColorGUI, config.TextColorGUI, target);
}
​

If all the values are reaching the other plugin and you verified that, then the issue would be with it. I will take a look myself as able.