Real-time visualization of assembly processes and sophisticated signal processing at the nanoscale are two challenging topics in photonic nanomaterials. Here, high-quality light-harvesting crystalline nanorods were developed by the coassembly of two polypyridyl Ir(III) and Ru(II) metallophosphors, behaving as the antenna chromophore and energy acceptor, respectively. By using a one-pot or stepwise growth condition, homogeneous and multiblock heterojunction nanorods were prepared, respectively. These nanostructures display multicolor phosphorescence from green to red due to the efficient triplet energy transfer and light-harvesting capability at low acceptor doping ratios. Heterojunction nanorods show gradient emission-color switches during different growth stages, in which the real-time stepwise assembly can be vividly visualized using fluorescence microscopy techniques. Triplet excitons were successfully manipulated in both homogeneous and heterojunction nanorods to realize waveguided green, orange, and red emissions and advanced photonic signal logics and encoding/decoding on single multiblock heterojunction nanorod.