Adding custom icons to activity bar?

I've got a couple of scripts which periodically display a message with notify-send after running their respective commands.

This works well, but I was wondering if there is a relatively easy/straightforward way to instead add an icon in the activity bar, beside the network and volume icons. That way, whenever one of my scripts run, the icon changes to something else to provide an indication of script status instead of using notify-send.

Script-wise, I'd be looking at doing something like:

if [ condition ]
then
ChangeToIcon1
elif [ condition ]
then
ChangeToIcon2
fi

Only problem is, I have no idea how to actually display and change icons, assuming there's a solution that wouldn't be overly time-consuming.

EDIT: Running 18.10, default DE (GNOME, I assume)