Displacement

In a displacement reaction of metals, a more reactive metal would displace a less reactive metal from its salt. The metal formed would show its own colour.

Example 1 : When a piece of zinc is put into copper(II) sulphate solution, zinc would displace copper from copper(II) sulphate solution. The colour of copper formed is brown, its own colour.
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Example 2 : When a piece of copper is put into silver nitrate solution, copper would displace silver from silver nitrate solution. The colour of silver formed is silvery, its own colour.
Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)

However, the metal formed is in powdered. Due to the absent of large smooth areas for reflection of light, the powder would look blacker than its own colour.
Therefore, when a small amount of metal (copper/silver) formed in the above either example, it looks black. When a greater amount of metal is formed, it looks brown/silvery, its own colour.