Why does conjuntion cause colour such as in the case of nitration of methyl benzoate?

"conjuntion"? I'm not sure what that means.

When you take one compound like Methyl Benzoate and functionalize the ring (as occurs in nitration), the electron cloud is dispersed or spread out from the benzene ring to the nitrate. The electrons won't interact with incoming radiation (photons) in the same manner. This causes differences in the wavelengths of light that is absorbed and thus, you can see a color change in the product.

Very frequently, if you create molecules with "conjugated double bonds", this will happen.

http://en.wikipedia.org/wiki/Conjugated_system

http://en.wikipedia.org/wiki/Chromophore