Carbon allotropes are in different colour. Eg. coke, graphite and charcoal in black, diamond in white, graphene in other color etc. What is the reason?
The bandgap of diamond is quite large, so photons of visible wavelength don't have enough energy to cause an excitation - therefore aren't absorbed. So diamond appears to be largely colourless.
Graphite, on the other hand, absorbs much of the visible spectrum and so appears to be black to human eyes.
There's a pretty nice little article about this here:
http://boingboing.net/2014/04/17/why-are-diamonds-clear-but-co.html
Yes, different crystal structures and different hybridisation of carbon lead to different electronic structures, hence the different electrical and optical properties. But only diamond is really very different from the others. The other black forms of carbon can be described by the repetition of the same hexagonal carbon pattern, with more or less disordered and defective structures, and the same sp2 hybridisation, so their electrical and optical properties are similar, although not identical (e.g. CNTs have slightly different electronic structure depending on their chirality, hence their possible semiconducting or metallic behaviour).
Alain
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