Answer: gold (yellow), copper (red), mercury vapors (orange), and in fact all metallic colors. The 3-dimensional system (red /green / blue, or equivalent) used to identify colors does not cover all colors. We should add at least one dimension, called the metallic index.
However, it is possible to simulate metallic colors using textures. See the results of one of my experiments (fractal texture) to emulate a copper (red copper) color:
On a different note, and since the same problem exists with printers as well, maybe bank notes should include colors such as gold or copper, to more easily detect counterfeit money. Not sure how expensive it would be to do this.