Question

Write the factors which are related to the colour of transition metal ions.

Answer 1

Transition metal ions are coloured due to presence of unpaired electrons which results in d-d transitions of these unpaired electrons.
Transition metal ions have incomplete $(n-1)$ d orbitals. A small amount of energy can promote one or more electrons from a lower energy level to higher energy level within d orbitals with same principal quantum number.
This small amount of energy comes in visible region. Transition metal ion absorbs energy from visible region and appears coloured.