What will happen if you dump blue dye into an actively used water softner?

I wanna play a prank on my friend, He has a water softner that are right in his front yard, easily accessible its as easy as riding a bike. my question is when this dye (non toxic) is introduced to the water softners, is there any longer term or dangerous side effects? I dont wanna damage anything although it would be hilarious to see him comew to school in really blue skin and clothes. But yea its a prank and we pull them on eachother occassionally. I just want it to be memerable. So is the dangerous? Or Just going to be inconveniently funny?