Without seeing the case we can't be sure, but probably it is described by
The phone case is black with silver decorations
this implies a case that after it was constructed had some aesthetically pleasing designs added, perhaps by painting or adhesive decals: The case was decorated.
The phone case is black with silver additions
Would describe the same physical situation but doesn't capture the thoughtful, artistic design implied by decorations.
The phone case is black with silver ornaments
Might apply, but ornaments has the implication of something such as wire-work or jewelry conspicuously encrusting the case, I doubt that you are describing such a case
The phone case is black with silver additives.
Implies that the silver was somehow mixed into the case's composition. More usually we apply this to chemical processes, such as those that produce processed foods. Something is mixed into the body of the substance, like anti-oxidents into some foodstuffs.
The phone case is black with silver accessories
When I think of accessories it's the small additions to a person's outfit that bring it life: well chosen scarves, belts, earrings, broaches, tie-pins ... in other words, optional add-ons.
I'm really not sure what kind of accessories a phone case might have. Perhaps a detachable cover? In which case that might be silver I suppose.