They dont scratch more easily, its basic physics, optics to be exact. Scratches are more visible in dark surfaces than light ones (considering the same material). Dark surfaces absorb more light and thus reflect less of it. For a smooth surface light should be reflected pretty uniformly and a scratch promotes a sharp change is the angle light does with the surface meaning what should be read by our eyes as a pretty uniform thing becomes messed up coming from the scratch area.
Light surfaces absorb much less energy and thus reflect much more compared to dark ones. That reflected light tho is much more diffuse (disorganized)/scathered. Since reflected light in light surfaces is already a mess, there is no visible change when the incoming light shines the scratched part.
In simpler words, the contrast is greater in dark surfaces. That doesn’t mean they scratch more easily, specially because for two finished that receive the same coating, scratch resistance should be the same.
Also, aluminium’s natural color is much closer to white than dark blue or orange so removing the coating shows a big contrast. If underneath the coating the material was black scratches would be much more apparent on a silver iPhone than on a black one.