Why are transition elemants are coloured ?

What do you mean with this?

 

The elements themselves are not especially colorful. Most of them are just silvery metals, only copper and gold are nicely colored.

 

Salts of these elements are colored though. The explanation for this is not easy in terms of high school physics. It has to do with the energy gap between lower and higher orbitals, used for bonding and used in ion formation. While for e.g. the alkali metals most light absorbtion is at very low wavelengths, this is for longer (and visible) wavelengts in the compounds of the transiton metals.

 

There is a reasonably sucesful theory, which helps explain (and sometimes even predict) colors of transition metal complexes. Try to understand that piece of information, but it is not really simple. The key to this is "crystal field theory". See e.g. the following Wiki page:

 

http://en.wikipedia.org/wiki/Crystal_field_theory

I don't say that transition metals are colored,

but their compounds / ions are colored

that is due to d-d transition of electrons..in them