[alokv28]

The color change of gold and CdSe particles with size are due to two different effects.

CdSe is a semiconductor, and reducing the CdSe nanoparticle size increases its band gap through a process called quantum confinement. For smaller particles, it requires a photon to have higher energy (i.e. smaller wavelength) to be absorbed.

Gold particles derive their color from the scattering mechanism mentioned in the parent comment.

[Osmium]

Completely agreed; I was just providing another example for how the colour of large objects does not necessarily have anything to do with the colour of small objects. Thanks for the added context :)