I never carry a provisional ball. It means you expect to hit a bad tee shot. Psychologically this is not good. I would rather do the walk of shame, and go get another from my bag.
I'm with Smiffy on this. It saves time walking back to bags which are often left near the fairway on some holes and can be offered to your partners in their time of crisis to stop them having to wander back.
I don't worry about the psychological impact having carried a spare ball for years.
I make sure that my original and provisional are marked completely separately and as it happens I've been carrying a multitude of brands recently so have made sure I use a completely different make. That way I'll declare a Titleist or whatever to start and my provisional may be a TM or a Srixon so no problem although I can see how others may perceive it. I'll always declare my second ball as a ....(whatever make) and the work provisional and will also remind what my first one is in case I need to play my second ball again before getting to the spot of my first one. At least my partners can have a good look then and know what ball they are looking at