why N2, H2 gas are colourless when visible light scatters it? why can't i see N2 and H2 coloured
The simple answer is that the electronic states of nitrogen and hydrogen are very far apart in energy. Both of these gases absorb in the vacuum ultraviolet region of the spectrum. The isolated gas molecules do not exhibit rotational or vibrational spectra, either.
There are emission bands between excited states of N2 and H2 that are in the visible region. You can see many of these if you pass an electric discharge through the gases at low pressure.