Painting car seperately, will there be color difference?

With metallic paint, just make sure you paint it hanging the way it's mounted on the car. So fi, don't hang the doors upright.

Don't think it doesn't matter much when painting non-metallics.

Yea metallics need to be sprayed with the car together to get the same look to every body panel. If you paint the car then the doors, hood etc once you put it together it might not match but non metallics will be fine to spray seperately .