Question

why O2 is colourless but F2 is colourful?​

Answer 1

In O2 photon absorbance frquency do not lie in the visible spectrum and hence you don't see any colour. Hence O2 is colourless.

In F2, the reason is that the colour is caused by absorption of a photon in the visible part of the spectrum. A spin change would correspond to a 'forbidden process' which has a low chance of occuring so leads to exceptionally weak absorption.

Explanation:

Hence, O2 is colourless and F2 is colourful.