I have never read a really good explanation of how opaque materials absorb light, but I think Amrit Sharma must be wrong in his explanation of the absorption of the color red. He talks about a material with a strong electronic resonance in the red spectrum, and supposes that those electronic vibrations are transformed down into lattice vibrations. I don't think that can be right.
If there is a strong electron resonance in the red spectrum, I'm pretty sure the material will look red. For light to be absorbed, it must be converted to mechanical lattice vibrations of a lower (much lower?) frequency. The mechanism for this is obscure...or at least, it is not easy to find a convincing explanation. The notion that this conversion is mediated by the electron resonances and is tempting, but I do not believe it is correct. There is no obvious mechanism for the down-conversion of frequencies.
I believe the true explanation must lie in the direct interaction of the incident light with the mechanical modes. We all know there are piezo-electric materials that exhibit a voltage when they are stressed, but it is easy to forget that every mechanical vibration generates some electric fields because the positive charged lattice ions will surely not drag the surrounding electrons along with them in perfect synch. Piezo-electric materials are special because the voltage builds up to a point where it can be measured externally, but almost all materials will surely have some internal piezo-electric effect. If there is, then it turns out there is a strong interaction between the incident light wave and the internal piezo-electric field when those two systems share the same wavelength. The mechanism is not much talked about, but it is actually the same mechanism responsible for the Compton effect. I explain this in a blogpost which you can find here.
The key point is to remember that in the Compton effect, you are actually removing energy from the light wave. That is something you don't get simply by stimulating the electron excitations...you store some energy, but that energy is eventually given back at the same frequency it came in. Not so the Compton effect. So I think this type of mechanism, with its direct interaction with a shared wavelength and an actual down-conversion of the incident light frequency, must be responsible for the absorption of light in opaque materials.