yes single stage no clear, I would agree with you about it looking like its been sanded through. What gets me is the car was primed then 3 coats of single stage was put on, so i am wondering why there is a change in shade between coats?
The reason for the ring (or as you describe - different shades) is because the metallic in the paint is dispersed separately in each coat, not necessarily throughout the entire multi-coat matrix, and also not necessarily perfectly uniform within an individual coat. As such, when you sand or polish through one coat you will see that ring or change in shade. You can also see color shifts even if you don't go all the way through from coat to coat.
The cause is the numbers of metallic particles, their distance relationship and orientation to the surface in the affected area. The uniformity of the metallic particles within any coat of paint is dependent on many factors... Paint reduction, air pressure at the gun, application technique - heavy/wet or not so heavy/wet, distance from gun to surface, dry time, and so on. As those parameters change, so do the paints reflective qualities, hence the color disparities.
Mike has an article about this in which he calls the phenomenon "Tiger Stripes". While stripes are certainly one way in which this condition presents itself, it may also be any random pattern of blotchiness, mottling, and/or color shifting.
Wet sanding and polishing of single stage metallic colors is as risky as it gets in the refinishing and detailing worlds.
Think of it like this... With a clear-coated finish you typically have 2 or 3 coats of clear to work with before you'd ever expose the base coat. With a single-stage metallic color you only have one coat before you reach the prior coat, and even less before you can start to alter the reflective qualities of the metallic even within the same coat of paint.
And then to compound matters, the final coat of single-stage metallics is often mist-coated to provide for uniform dispersement of the metallic particles at the paint's surface. So now the metallic particles are very close to the surface and any sanding or polishing can disrupt that uniformity in a heartbeat.
The preceding explanation applies to "refinished" metallic colors.
In the old days of single-stage factory paints these issues, while they did exist, were not as prominent. The paint process at the factory resulted in a more even distribution of the metallic throughout the entire paint matrix. Even so, sanding and/or polishing could create similar issues with the factory finish, but not to the same degree as with refinished parts.
If you want a glass smooth finish with a metallic color, you have to clear-coat it. It's simply not possible to sand and polish a single-stage metallic color perfectly smooth without the likelihood of issues like you are experiencing. Single-stage metallics can only be lightly polished, and even that poses some risk.
