The origin of colors of nanoparticles suspension are similar to the one that cause the sky to be blue and setting sun to be red… Light scattering intensity is a very strong function of light wavelength to particle circumference ratio. (http://www.biomedlab.ichip.pw.edu.pl/content/view/33/8/lang,english/)
The sky is blue because it scatters blue light, sun is red because blue color was scattered away stronger then red, removed from the spectra.
If nanoparticle size distribution is fairly narrow they scatter certain light color away, or absorb energy and remove it from the spectra. In the case of silver nanoparticles, we also have to take into account surface plasmon phenomenon, nanoparticles seems to be “bigger” since electrons are present also a small distance above their surface and take part in the absorption/scattering of light. If something is adsorbed at the surface, they change color… Gold nanoparticles of 50nm are strongly red and were used to stain glass in middle ages (medieval nanotechnology).
What Tomasz said is one possibility. You could also check lots of related question in this ResearchGate facing similar questions:
https://www.researchgate.net/post/How_does_one_explain_the_relationship_between_particle_size_and_its_color_in_solution
https://www.researchgate.net/post/How_can_the_particle_size_of_a_metal_nanoparticles_control_the_UV-vis_behavior
https://www.researchgate.net/post/Particle_size
https://www.researchgate.net/post/Can_anyone_tell_me_that_color_change_origin_of_synthesized_nanoparticles_in_the_different_sizes
https://www.researchgate.net/post/Will_the_absorption_peaks_of_UV-visible_spectra_of_CdSe_quantum_dots_shift_to_longer_or_shorter_wavelengths_with_increased_particle_sizes
And also quantum confinement related entries:
https://www.researchgate.net/post/What_is_Quantum_confinement_effect
http://en.wikipedia.org/wiki/Potential_well
https://www.youtube.com/watch?v=7f1MW4QowYs
Hope it helps...
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