Solution of copper sulphate is deep blue. When diluted with warm water, changes to paler blue. Explain this in terms of the particle theory of...

When white light shines through a solution or other
transparent material, the wavelengths that are NOT absorbed are the ones that you see - 
in the case of Copper II Sulphate, the blue wavelengths are being both transmitted and
reflected, while the other colors (primarily red wavelengths in this case) are absorbed
by the solution.This absorption occurs because some of the valence electrons (D block in
the case of transition metals like copper) are able to absorb specific wavelengths of
light which kick them to a higher energy state. As you dilute the solution, these ions
are spread farther apart, and more light passes through without encountering a copper
ion; the additional  light transmitted is of all different wavelengths, thus adding up
to "white" as we perceive it visually.

The Beer-Lambert Law
states the mathematical relationship among absorption, transmission, and solution
concentration. Basically it states that the "optical density" of a solution is directly
proportional to its concentration. I have included a link to a photometry site here that
gives the Law and its mathematical derivation if you need them.