Colloidal silver can be almost any spectral colour. By varying the size and shape of a silver nanoparticle you can adjust the peak of it's reflectance right across the visible spectrum. It's a hot topic in current nanoscience.
First pic: a mixture of silver nanoparticles of various sizes and shapes as seen in an optical microscope in darkfield mode - you are seeing backscattered light here. (from Orendorff et al, Small 2, p636, 2006)
Second pic: Peak reflectances for various silver shapes and sizes. (from Mock et al, J. Chem.Phys. 116, p6755, 2002)
The second paper by Mock and Co. is the one to look for if you want a single reference. Otherwise a google search on "plasmon resonance nanoparticle" will turn up lots of relevant hits.
In the 50s Robert Bensley claimed to be able to tune the production of silver particles in photographic emulsions so as to produce full colour images. Current research makes it clear why this might work, but it is not at all clear that Bensley's relatively simple method offers enough control to work reliably. Despite publishing in Science, I have yet to find a single reference to the work in the published literature, or even any followup experiments. Science 112, p553, 1950 if you want to look it up.