Why does D- block elements form coloured compounds?

Answer 1

Due to splitting of degenerate orbitals of d-subshell.
The dxy, dxz and dyz orbitals will be lower energy than the dz2 and dx2-y2, which will have higher energy, because the former group are further from the ligands than the latter. In a tetrahedral crystal field splitting, the lower energy orbitals will be dz2 and dx2-y2, and the higher energy orbitals will be dxy, dxz and dyz - the opposite way round to the octahedral case.
The size of the gap Δ between the two sets of orbitals depends on several factors, including the ligands. Some ligands always produce a small value of Δ, and some always give a large value