I once saw a pair of blue lights, one above the other, on a post at the end of a pier. This was on a private pier along the ICW. I assume that this was to avoid any possible confusion, because blue lights aren't used in any aids to navigation, but white, red and yellow lights are. If you used two posts, a taller one behind a shorter one, you could also derive your relative bearing from the apparent offset or alignment of the two lights. But I'm not sure what the regulations are about private pier lighting and such.