I've done this, expecting a nightmare, but the results (colour match wise) have always been spot on. A few things which I think make a big difference.
1) A good match to start with. I use paints4u, they get the colour match spot on, every time.
2) A good can. The ones from the people mentioned about have a fan nozzle, which mimics a spray gun much more than a cone type nozzle you might get.
3) Leave the can in warm water before spraying, and shake it a lot, not just a quick rattle.
4) Keep a consistent distance from the surface! This is one of the biggest ones for me. Read the can, get a tape measure and see how far e.g 15cm actually is from the surface and visualise it while you are spraying. And always at 90 degrees to the surface (no wrist flicking!)
5) Build up your layers gradually. Another important on. This way you can stop when the colour matchest. As said, I always though the colour match would be a nightmare, but I just build the layers until it matches, then stop. This might be the wrong official method, but it works for me.
6) Don't sand down the colour coat, before clear. I understand this knocks the metallic flakes out of where they naturally fell.
Blending clear coat, now that is a whole different question. Still havnt got that down.
