“we know about absorption & emission spectra. We get colours; so what is orgin of colurs? why the atoms emit colour spectra”
Light is emitted when an excited atom returns to its ground state. When excited, the electrons jump from lower level to higher level absorbing the energy equal to the difference in energy levels corresponding to the orbits. When these electrons come back to the original states the energy is emitted.
As we know E = hν=hc/λ
Accordingly light of different wavelengths (and hence different colours) are emitted depending on the energy difference between the levels of transition.
For more details, please refer to these links.