Why Fe(CO)5 is colourless while Fe(bipy)(CO)3 is intensely purple in colour ?

The intense colour of the latter complex is strongly suggestive of a charge transfer transition and since the metal is already fully reduced (zero oxidation state), it is highly likely that this involves a MLCT transition. The π* levels of the bipy or CO ligands are possible acceptors but the fact that Fe(CO)₅ doesn’t show this colour suggests that it is the bipy π* levels that are involved in Fe(bipy)(CO)₃. Since there should be MLCT transitions to the CO π* levels as well, we assume that the lack of colour for Fe(CO)₅ means that these transitions fall in the UV rather than the visible.

Why all the tetrahedral Complexes are high spin Complexes ?

The magnitude of crystal field splitting energy (CFSE) in tetrahedral Complexes is quite small and it is always less than the pairing energy. Due to this reason pairing of electron is energetically unfavorable. Thus all the tetrahedral Complexes are high spin Complexes. In fact no tetrahedral Complex with low spin has been found to exist.