I'm not sure about modern metallics but in my day it depended on how the paint was applied. If it was done right, sanding would change the color because the metallic would settle under the pigment - paint has three components: vehicle, binder and pigment. Metallics add microscopic pieces of bright flakes of metal or something else. Sanding would thin out the pigment layer and consequently the microscopic specs of metallic would appear lighter.
We always put a lot of clear over metallics and just wet sanded and buffed that. It's okay to scuff the surface, but avoid deep sanding.
At least that's my experience.
