EDIT BY ME: One more thing I would like to add is that previous attempts to control coontail didn't work when that coontail had grown to several inches below the surface. I'm not sure if it was the previous dyes I was using, or the coontail's ability to get unfiltered sunlight, but for whatever reason, dye didn't work. Dye doesn't bother my water primrose, so my assumption is it was the sunlight.
Your observation is a concept that bears repeating. Suitable lake-dyes will generally "work" only as a deterrent/preventative measure; meaning they should be applied long before a weed or algae problem becomes evident or established. Several feet of water-column depth are generally required for light-filtration to occur; with the relative depth being determined by the dye-dosage. If submerged plant-growth has already ascended to near the surface - or is on the surface (primrose) - it's unlikely that a dye-treatment at that point will offer any benefit unless the plant's biomass is significantly lowered to a greater depth by some other means (ie. mechanically or with a herbicide treatment).
