For externally synthesized silver nanoparticles the silver ion solution generally becomes brownish in color, but this browning reaction may be absent.  For fungi that synthesize intracellular silver nanoparticles, the hyphae darken to a brownish color while the solution remains clear. In both cases the browning reaction is attributed to the surface plasmon resonance of the metallic nanoparticles. For external gold nanoparticle production, the solution color can vary depending on the size of the gold nanoparticles; smaller particles appear pink while large particles appear purple. Intracellular gold nanoparticle synthesis typically turns the hyphae purple while the solution remains clear. Externally synthesized cadmium sulfide nanoparticles were reported to make the solution color appear bright yellow.