We have directly observed the ring formation of colloidal particles of 1 mu m diameter at the contact lines of air, water, and oil using a laser scanning confocal microscope. Colloidal rings form and grow through the transport of particles induced by capillary flow due to water evaporation. In addition, we observe the sudden "jump in" of particles into the ring and the "depletion" of particles in the ring. Particle-tracking experiment shows that the particles within the ring exhibit 1D-like motion along the circular ring geometry, and the pair correlation function of the ring configuration suggests an equilibrium interparticle distance of approximately 2.8 mu m. It is also found that the structure and formation speed of the colloidal rings can be controlled by accelerating water evaporation by the addition of methanol as a cosolvent.