I have used Dr. Colorchip twice. Both times on metallic paint. On one car the color was noticeably lighter, on the other it was noticeably darker. And I made sure that the paint was very well stirred. It did work as advertised, other that the color disparity.
I do find their recommended technique of smearing the paint blob with a gloved finger to be just plain dumb. I had a little leftover silicone squeegee that was supplied with a cell phone screen protector kit and used that instead. It worked very well. They should supply something like that with their basic kit considering what it costs. You may need two applications with deep chips. It's better not to try and fill in a deep chip with a thick blob, but to use a fine artists brush to get a base coat down into the chip below the surface, let it dry overnight and then add another coat per the directions and bring it up even with the surface.