Keep in mind the only purpose of anti fouling bottom paint is to retard the growth of marine plant and animal stuff on your bottom. It will keep working as long as whatever the active ingredient used to do that is still present. If you only had minimal growth in the period of time you were in water, then it is still working. With a 16 that can be hauled, trailered and cleaned at any time I would push it to its limits. The only down side to "overusing" the paint job is you have to haul and clean to remove the growth and keep the boat from slowing down to a crawl when you try to sail it.
My 23 stays in the water all the time, only hauled for short periods for maintenance and a bottom job. I use Blue Water Marine SCS 45, similar to the Interlux you mentioned, and push three years out of it by doing in water cleanings in the shallows at least once a year.
I don't think a touch up does much good, although if you have the paint handy, a full coat six inches or so from the waterline down will help. Growth of barnacles and marine plant life tends to concentrate just below the waterline.
The color change is a good idea. I have epoxy gray barrier covered with black initial ablative anti fouling, with red anti fouling over the black. Now when I see any black it lets me know that the antifouling top coat has ablated off.