When we add extra metal to the lattice, it provide extra electrons to the lattice and the anions of the lattice leave their lattice points . These vacant sites are occupied by the electrons. And the free electron can easily absorb energy and can jump to higher level.
As these energy transitions are in visible spectrum of light so the crystal appears to be coloured.
For example in case of NaCl crystal, when we heat it the crystal appear yellowish.
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