Silver gets fat and thin: Ag nanoparticles photocatalytically deposited on rutile TiO2 single crystals exhibit multicolor photochromism. Under monochromatic visible light, particles resonant with the light are photoelectrochemically dissolved, and re-deposited on the TiO2 substrate or non-resonant particles. These size-selective dissolution and growth are responsible for the wavelength-dependent color changes.