How should I prep for a color change over existing good paint?

JDMopar said
If yours is a bigger one, like 1.7 or 1.8 that is for primer, you can get one of the cheap Harbor Freight purple guns that is 1.4

I do have one of the purple guns, a Bostitch, and a Nieko. I don't remember the tip sizes of the other two.

Kendog 170 said
If your talking Base coat clearcoat I sand the adjacent panel with 1000 grit. Paint said part in base color then blend color partway into adjacent panel, then clearcoat all. No blender needed.

One of her coworkers painted her bumper a while back, but it looks pretty rough. Maybe I'll test out my skills and redo her bumper while I'm at it. If I do that, would it make sense to just shoot clear over the entire car?