How are the color of an compound and it's constituent elements related?

[A@bhishek]

Suppose the color of some elements is red and blue but after the reaction between them a new compound formed with yellow color .so on what factor does the the color of compound depend?
I think it may depend depend upon the number of electron transferred and the remaining one...

 

[Asteropaeus]

It depends on the energy difference between molecular orbitals. When a photon hits electron density, an electron can transition to a higher energy orbital. The photon disappears.

Compounds that absorb light in the visible spectrum will have a colour. The colour will be whatever you get when you take the whole spectrum and remove the absorbed part of the spectrum.

When molecular orbitals change as the molecule rearranges its electrons as new bonds are formed, so change the energy difference between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO). Hence, they have a different spectrum.

Only certain compounds have HOMO to LUMO transitions with energy that corresponds to the visible spectrum. So only these certain molecules have colour.