I know that elements with unpaired electrons are coloured especially the d-block element which can be explained by crystal field theory. But, why can't element with paired electrons give colour, is it because the energy gap is too large ?

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    $\begingroup$ It's not about whether the electrons are paired or not. It has to do with whether the electronic transitions in the molecule lie within the visible spectrum $\endgroup$ – orthocresol Sep 30 '15 at 15:20
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    $\begingroup$ to give you counterexamples, Ni(II) in its dimethylglyoxime complex has 8 d electrons and all of them are paired but the complex is red. On the other hand $\ce{O2}$ gas has 2 unpaired electrons but it's colourless. $\endgroup$ – orthocresol Sep 30 '15 at 15:30
  • $\begingroup$ @orthocresol actually, liquid oxygen has a blueish colour, or so I heard. $\endgroup$ – Jan Sep 30 '15 at 16:36
  • $\begingroup$ @Jan I've seen that before in a demonstration... which is why I said O2 gas :D Anyway I went to read up. Turns out that the blue colour is because of two O2 molecules being excited simultaneously, which has a very low probability of occurring in the gas phase (hence colourless). Probably wasn't the best example to use but I guess it's passable. $\endgroup$ – orthocresol Sep 30 '15 at 16:58
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    $\begingroup$ @orthocresol Disregard me, I overread the 'gas' in there ^^' But on that note: chlorine gas is greenish, bromine is brown, iodine is purple, liquid chlorine is yellow. $\endgroup$ – Jan Sep 30 '15 at 16:59

Your assumption is wrong. There are elements with paired electrons which are coloured.

Colour is a product of electronic transitions in atoms or molecules. If those transitions match the energy of a photon of visible light, then the substance will be coloured (though there are some other rules about how intense the colour will actually be).

Some elements do show colour. Halogens, for example, exist as diatomic molecules in their native state and are all coloured. Chlorine is a greenish gas and bromine is an intensely red liquid. Sulphur is usually found as a yellow solid. All have paired electrons.

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